Sleep is a
naturalNatural can refer to various topics within science and mathematics, music, and other areas.In science and mathematics, natural may refer to:* Natural transformation, category theory in mathematics...
ly recurring state of relatively suspended sensory and motor activity, characterized by total or partial unconsciousness and the inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than
hibernationHibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve energy, especially during winter when food is short, tapping energy reserves, body fat, at a slow rate...
or
comaIn medicine, a coma is a profound state of unconsciousness. A comatose person cannot be awakened, fails to respond normally to pain or light, does not have sleep-wake cycles, and does not take voluntary actions....
. It is observed in all mammals, including humans, all birds, and many reptiles, amphibians, and
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
. In humans, other mammals, and a substantial majority of other animals that have been studied (such as some species of fish, birds, ants, and
fruit fliesDrosophilidae is a diverse, cosmopolitan family of flies, including the genus Drosophila, which includes fruit flies. The best known species is Drosophila melanogaster that is used extensively for studies concerning genetics, development, physiology, ecology, behaviour, etc...
), regular sleep is essential for survival.
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.
Stages of sleep
In mammals and birds, sleep is divided into two broad types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The
American Academy of Sleep MedicineThe American Academy of Sleep Medicine is a United States professional society for the medical subspecialty of sleep medicine. It was established in 1975. The is the only professional society that is dedicated exclusively to the medical subspecialty of sleep medicine...
(AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called
deltaA delta wave is a high amplitude brain wave with a frequency of 1–4 Hertz which can be recorded with an electroencephalogram and is usually associated with slow-wave sleep....
, or
slow-wave, sleepSlow-wave sleep , often referred to as deep sleep, consists of stages three and four of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3...
(SWS).
Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) early in the night, while the proportion of REM sleep increases later in the night and just before natural awakening.
The stages of sleep were first described in 1937 by
Alfred Lee LoomisAlfred Lee Loomis was an American attorney, investment banker, physicist, philanthropist and patron of scientific research. He established the Loomis Laboratory in Tuxedo Park, New York, and his role in the development of radar is considered instrumental in the Allied victory in World War II...
and his coworkers, who separated the different EEG features of sleep into five levels (A to E), which represented the spectrum from wakefulness to deep sleep. In 1953, REM sleep was discovered as distinct, and thus William Dement and
Nathaniel KleitmanNathaniel Kleitman was Professor Emeritus in Physiology at the University of Chicago. Author of the seminal 1939 book Sleep and Wakefulness, he is recognized as the father of American sleep research...
reclassified sleep into four NREM stages and REM. The staging criteria were standardized in 1968 by
Allan RechtschaffenAllan Rechtschaffen is a noted pioneer in the field of sleep research whose work includes some of the first laboratory studies of insomnia, narcolepsy, and napping....
and Anthony Kales in the "R&K sleep scoring manual." In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.
In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system. The review culminated in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. The revised scoring was published in 2007 as
The AASM Manual for the Scoring of Sleep and Associated Events. Arousals and respiratory, cardiac, and movement events were also added.
Sleep stages and other characteristics of sleep are commonly assessed by
polysomnographyPolysomnography , also known as a sleep study, is a multi-parametric test used in the study of sleep and as a diagnostic tool in sleep medicine. The test result is called a polysomnogram, also abbreviated PSG...
in a specialized sleep laboratory. Measurements taken include
electroencephalographyElectroencephalography is the recording of electrical activity along the scalp produced by the firing of neurons within the brain . In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20–40 minutes, as recorded from...
(EEG) of brain waves,
electrooculographyElectrooculography is a technique for measuring the resting potential of the retina. The resulting signal is called the electrooculogram. The main applications are in ophthalmological diagnosis and in recording eye movements...
(EOG) of eye movements, and
electromyographyElectromyography is a technique for evaluating and recording the activation signal of muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle cells when these...
(EMG) of
skeletal muscleSkeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system. It is one of three major muscle types, the others being cardiac and smooth muscle. As its name suggests, skeletal muscle is linked to bone by bundles of collagen fibers known as...
activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average, and each stage may have a distinct physiological function. Drugs such as sleeping pills and
alcoholic beverageAn alcoholic beverage is a drink that contains ethanol . Alcoholic beverages are divided into three general classes: beers, wines, and spirits....
s can suppress certain stages of sleep, leading to
sleep deprivationSleep deprivation, a sleep disorder characterized by having too little sleep, can be either chronic or acute. Long-term sleep deprivation causes death in lab animals...
. This can result in sleep that exhibits
loss of consciousnessUnconsciousness, more appropriately referred to as loss of consciousness or lack of consciousness, is a dramatic alteration of mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is an illustration...
but does not fulfill its physiological functions (i.e., one may still feel tired after apparently sufficient sleep). REM and slow-wave sleep are both homeostatically driven; people and most animals selectively deprived of one of these stages will rebound once uninhibited sleep is allowed. This finding suggests that both of these stages are essential.
NREM sleep
According to the 2007 AASM standards, NREM consists of three stages. There is relatively little dreaming in NREM.
Stage N1 refers to the transition of the brain from
alpha wavesAlpha Waves is an early 3D game that combines labyrinthine exploration with platform gameplay. By most definitions of the genre it could be considered to be the first 3D platform game, released in 1990, 6 years before the genre's seminal classic Super Mario 64...
having a frequency of 8 to 13
HzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
(common in the awake state) to
theta wavesA theta rhythm is an oscillatory pattern in EEG signals recorded either from inside the brain or from electrodes glued to the scalp. Two types of theta rhythm have been described...
having a frequency of 4 to 7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and
hypnic jerkA hypnic or hypnagogic jerk is an involuntary myoclonic twitch which occurs during hypnagogia, just as the subject is beginning to fall asleep...
s, also known as positive
myoclonusMyoclonus is brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign and, generally, is not a diagnosis of a disease. The myoclonic twitches are usually caused by sudden muscle contractions; they also can result from brief lapses of contraction. Contractions...
, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage, which can be troublesome to them. During N1, the subject loses some
muscle toneIn physiology, medicine, and anatomy, muscle tone is the continuous and passive partial contraction of the muscles. It helps maintain posture, and it declines during REM sleep...
and most conscious awareness of the external environment.
Stage N2 is characterized by
sleep spindleA sleep spindle is a burst of brain activity visible on an EEG that occurs during stage 2 sleep. It consists of 12-16 Hz waves that occur for 0.5 to 1.5 seconds.-Function:...
s ranging from 11 to 16 Hz (most commonly 12–14 Hz) and
K-complexA K-complex is an electroencephalography waveform that occurs during stage 2 of NREM sleep. It is the "largest event in healthy human EEG". It consists of a brief negative high-voltage peak, usually greater than 100 µV, followed by a slower positive complex around 350 and 550 ms and at 900ms a...
es. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45% to 55% of total sleep in adults.
Stage N3 (deep or
slow-wave sleepSlow-wave sleep , often referred to as deep sleep, consists of stages three and four of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3...
) is characterized by the presence of 20%
delta waveA delta wave is a high amplitude brain wave with a frequency of 1–4 Hertz which can be recorded with an electroencephalogram and is usually associated with slow-wave sleep....
s ranging from 0.5 to 2 Hz and having a peak-to-peak amplitude > 75 μV(EEG standards define delta waves to be from 0 – 4 Hz; but, sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have traditionally defined them at a maximum of 2 Hz). This is the stage in which such
parasomniaFor the 2008 horror film, see Parasomnia Parasomnias are a category of sleep disorders that involve abnormal and unnatural movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or arousal from sleep...
s as night terrors,
bedwettingBedwetting is involuntary urination while asleep after the age at which bladder control would normally be anticipated. The medical term for this condition is "nocturnal enuresis." Primary nocturnal enuresis is when a child has not yet stayed dry on a regular basis...
,
sleepwalkingSleepwalking is a parasomnia or sleep disorder where the sufferer engages in activities that are normally associated with wakefulness while he or she is asleep or in a sleep-like state...
, and
sleep-talkingSomniloquy or sleep-talking is a parasomnia that refers to talking aloud in one's sleep. It can be quite loud, ranging from simple sounds to long speeches, and can occur many times during sleep...
occur.
REM sleep
Rapid eye movement sleep, or REM sleep, accounts for 20%–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.
Timing
Sleep timing is controlled by the
circadian clockA circadian rhythm is a roughly-24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria...
, sleep-wake
homeostasisHomeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition...
, and in humans, within certain bounds, willed behavior. The circadian clock —- an inner timekeeping, temperature-fluctuating, enzyme-controlling device —- works in tandem with
adenosineAdenosine is a nucleoside composed of a molecule of adenine attached to a ribose sugar molecule moiety via a β-N
9-glycosidic bond....
, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone
melatoninMelatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring hormone found in animals and in some other living organisms, including algae. Circulating levels vary in a daily cycle, and melatonin is important in the regulation of the circadian rhythms of several...
and a gradual decrease in core
body temperatureThermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
. The timing is affected by one's
chronotypeChronotype is an attribute of human beings reflecting whether they are alert and prefer to be active early or late in the day. The continuum is often referred to as “morningness/eveningness” or “larks” and “owls” where morning people wake up early and are most alert in the first part of the day,...
. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.
Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than their normal waking time, even if moderately sleep-deprived.
Sleep duration is affected by
circadian rhythmA circadian rhythm is a roughly-24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria...
which is regulated by a gene named DEC2. Some people have a mutation of this gene; they sleep two hours less than normal. Neurology professor Ying-Hui Fu and his colleagues bred mice that carried the DEC2 mutation and slept less than normal mice.
Adult
The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual's
circadian rhythmA circadian rhythm is a roughly-24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria...
s. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day; one should be asleep at least six hours before the lowest body temperature. The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:
- maximum concentration of the hormone melatonin, and
- minimum core body temperature.
The
National Sleep FoundationThe National Sleep Foundation is an independent nonprofit organization in the USA whose objectives are to improve public health and safety by achieving understanding of sleep and sleep disorders, and by supporting sleep-related education, research, and advocacy.Established in 1990, NSF relies on...
in the United States maintains that seven to nine hours of sleep for adult humans is optimal and that sufficient sleep benefits alertness, memory,
problem solvingProblem solving is a mental process and is part of the larger problem process that includes problem finding and problem shaping. Consideredthe most complex of all intellectual functions, problem solving has been defined as higher-order cognitive process that requires the modulation and control of...
, and overall health, as well as reducing the risk of accidents. A widely publicized 2003 study performed at
the University of PennsylvaniaThe University of Pennsylvania is a private research university located in Philadelphia, Pennsylvania, USA. Penn is the fourth-oldest institution of higher education in the United States, and is one of several institutions that claims to have been the first university in America...
School of Medicine demonstrated that cognitive performance declines with six or fewer hours of sleep.
A
University of California, San DiegoThe University of California, San Diego is a public research university located in La Jolla, San Diego, California, United States...
, psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night. Another study of sleep duration and mortality risk in women showed similar results. Other studies show that "sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality," though this study suggests the cause is probably other factors such as depression and socioeconomic status, which would correlate statistically. It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake after less sleep naturally, rather than those who use an alarm.
Researchers at the
University of WarwickThe University of Warwick is a British campus university located on the outskirts of Coventry, West Midlands, England. It was established in 1965 as part of a government initiative to expand access to higher education, and in 2000 Warwick Medical School was opened as part of an initiative to train...
and
University College LondonUniversity College London is a British university institution and a constituent college of the University of London, based primarily in Bloomsbury, London...
have found that lack of sleep can more than double the risk of death from
cardiovascular diseaseCardiovascular disease or cardiovascular diseases is the class of diseases that involve the heart or blood vessels ....
, but that too much sleep can also be associated with a doubling of the risk of death, though not primarily from cardiovascular disease. Professor Francesco Cappuccio said, "Short sleep has been shown to be a
risk factorA risk factor is a variable associated with an increased risk of disease or infection. Risk factors are correlational and not necessarily causal, because correlation does not imply causation...
for weight gain,
hypertensionHypertension is a chronic medical condition in which the blood pressure is elevated. It is also referred to as high blood pressure or shortened to HT, HTN or HPN. The word "hypertension", by itself, normally refers to systemic, arterial hypertension.Hypertension can be classified as either...
, and
Type 2 diabetesDiabetes mellitus type 2 or type 2 diabetes is a disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency...
, sometimes leading to mortality; but in contrast to the short sleep-mortality association, it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status, and cancer-related fatigue. …In terms of prevention, our findings indicate that consistently sleeping around seven hours per night is optimal for health, and a sustained reduction may predispose to ill health."
Furthermore, sleep difficulties are closely associated with psychiatric disorders such as depression,
alcoholismAlcoholism is a term with multiple and sometimes conflicting definitions. In common and historic usage, alcoholism is any condition that results in the continued consumption of alcoholic beverages, despite health problems and negative social consequences...
, and
bipolar disorderBipolar disorder, also known as manic depressive disorder, manic depression or bipolar affective disorder, is a serious mental disorder that describes a category of mood disorders defined by the presence of one or more episodes of abnormally elevated mood clinically referred to as mania or, if...
. Up to 90% of adults with depression are found to have sleep difficulties. Dysregulation found on EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.
Hours by age
Children need more sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. A newborn baby spends almost 9 hours a day in REM sleep. By the age of five or so, only slightly over two hours is spent in REM.
| Age and condition |
Average amount of sleep per day |
| Newborn |
up to 18 hours |
| 1–12 months |
14–18 hours |
| 1–3 years |
12–15 hours |
| 3–5 years |
11–13 hours |
| 5–12 years |
9–11 hours |
| Adolescents |
9–10 hours |
| Adults, including elderly |
7–8 (+) hours |
| Pregnant women |
8 (+) hours |
Sleep debt
Sleep debt is the effect of not getting enough rest and sleep; a large debt causes mental, emotional, and physical fatigue. It is unclear why a lack of sleep causes irritability; however, theories are emerging that suggest if the body produces insufficient
cortisolCortisol is a corticosteroid hormone or glucocorticoid produced by the adrenal cortex, that is part of the adrenal gland . It is usually referred to as the "stress hormone" as it is involved in response to stress and anxiety, controlled by CRH...
during deep sleep, it can have negative effects on the alertness and emotions of a person during the day.
Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.
Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the
industrialized worldThe term developed country is used to describe countries that have a high level of development according to some criteria. Which criteria, and which countries are classified as being developed, is a contentious issue and there is fierce debate about this. Economic criteria have tended to dominate...
in recent decades. It is likely that children are sleeping less than previously in
Western societiesThe Western world, also known as the West and the Occident , is a term that can have multiple meanings depending on its context...
.
Genetics
A considerable amount of sleep-related behavior is apparently hard-wired into human biology - humans in all cultures get tired, require sleep for good health, and have similar symptoms when sleep deprived. Scientific research has identified some genetic variations, including:
- A mutation that moves consolidated sleep earlier, resulting in a sleep cycle from 7:30pm to 3:30am.
- A mutation in BHLHB3
Class E basic helix-loop-helix protein 41 is a protein that in humans is encoded by the BHLHE41 gene.One study has found that genetic variation in the gene may affect sleep time in mice....
which apparently reduces the amount sleep needed for healthy living to 6 hours from 8.
Functions
The multiple theories proposed to explain the function of sleep reflect the as-yet incomplete understanding of the subject. It is likely that sleep evolved to fulfill some primeval function and has taken over multiple functions over time—just as with, for example, the
larynxThe larynx , colloquially known as the voicebox, is an organ in the neck of mammals involved in protection of the trachea and sound production...
which today controls the passage of food and air and provides phonation for communicating and other social purposes.
It has been pointed out that, if sleep were not essential, one would expect to find 1) animal species that do not sleep at all, 2) animals that do not need recovery sleep when they stay awake longer than usual, and 3) animals that suffer no serious consequences as a result of lack of sleep. No animals have been found to date that satisfy any of these criteria.
Some of the many proposed functions of sleep are as follows.
Restoration
Wound healingWound healing, or wound repair, is an intricate process in which the skin repairs itself after injury.[Nguyen, D.T., Orgill D.P., Murphy G.F. . Chapter 4: The pathophysiologic basis for wound healing and cutaneous regeneration. Biomaterials For Treating Skin Loss. CRC Press &...]
has been shown to be affected by sleep. A study conducted by Gumustekin et al. in 2004 shows sleep deprivation hindering the
healingHealing is the act or process of curing or of restoring to health. Assessed physically, healing is the process by which the cells in the body regenerate and repair to reduce the size of a damaged or necrotic area....
of burns on rats.
It has been shown that sleep deprivation affects the
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
. In a study by Zager et al. in 2007, rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats'
blood testA blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a needle, or via fingerprick....
s indicated a 20% decrease in
white blood cellWhite blood cells , or leukocytes , are cells of the immune system defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...
count, a significant change in the immune system. It is now possible to state that "sleep loss impairs immune function and immune challenge alters sleep," and it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts.
It has yet to be proven that sleep duration affects
somaticThe term somatic refers to cells of the body, rather than gametes . In humans, somatic cells contain two copies of each chromosome , whereas gametes only contain one copy of each chromosome...
growth. One study by Jenni et al. in 2007 recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." It has been shown that sleep—more specifically, slow-wave sleep (SWS)—does affect
growth hormoneGrowth hormone is a protein-based poly-peptide hormone. It stimulates growth and cell reproduction and regeneration in humans and other animals. It is a 191-amino acid, single-chain polypeptide hormone that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of...
levels in adult men. During eight hours' sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.
There are multiple arguments supporting the restorative function of sleep. We are rested after sleeping, and it is natural to assume that this is a basic purpose of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, in general,
inversely relatedAn inverse or negative relationship is a mathematical relationship in which one variable, say y, decreases as another, say x, increases. For a linear relation, this can be expressed as y = a-bx, where -b is a constant value less than zero and a is a constant...
to animal size and directly related to
basal metabolic rateBasal metabolic rate , and the closely related resting metabolic rate , is the amount of energy expended while at rest in a [neutrally temperate environment, in the post-absorptive state...
. Rats with a very high basal metabolic rate sleep for up to 14 hours a day, whereas elephants and giraffes with lower BMRs sleep only 3–4 hours per day.
Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example,
hibernatingHibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve energy, especially during winter when food is short, tapping energy reserves, body fat, at a slow rate...
animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else. Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass,
hypothermiaHypothermia is a condition in which an organism's temperature drops below that required for normal metabolism and body functions. In warm-blooded animals, core body temperature is maintained near a constant level through biologic homeostasis or thermoregulation...
, and eventually, septicemia and death.
Anabolic/catabolic
Non-REM sleep may be an
anabolicAnabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy. One way of categorizing metabolic processes, whether at the cellular, organ or organism level is as 'anabolic' or as 'catabolic', which is the opposite...
state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems (with some exceptions). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and reproduces.
Ontogenesis
According to the
ontogeneticOntogeny describes the origin and the development of an organism from the fertilized egg to its mature form...
hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al., 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).
REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses, yet without any motor consequences that may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.
However, this does not explain why older adults still need REM sleep.
Aquatic mammalMarine mammals are a diverse group of roughly 120 species of mammal that are primarily ocean-dwelling or depend on the ocean for food. They include the cetaceans , the sirenians , the pinnipeds , and several otters...
infants do not have REM sleep in infancy; REM sleep in those animals increases as they age.
Memory processing
Scientists have shown numerous ways in which sleep is related to
memoryIn psychology, memory is an organism's mental ability to store, retain, and recall information. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing the memory....
. In a study conducted by Turner, Drummond, Salamat, and Brown,
working memoryWorking memory is a theoretical construct within cognitive psychology as to the structures and processes used for temporarily storing and manipulating information in short-term memory. Many theories exist both as to the theoretical structure of working memory as well as to the role of specific...
was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level
cognitive functionsIn some forms of psychological testing, particularly those related to the Myers-Briggs Type Indicator, the cognitive functions are defined as different ways of perceiving and judging the world...
such as
decision makingDecision making can be regarded as an outcome of mental processes leading to the selection of a course of action among several alternatives. Every decision making process produces a final choice...
,
reasoningReasoning is the cognitive process of looking for reasons for beliefs, conclusions, actions or feelings.Humans have the ability to engage in reasoning about their own reasoning. Different forms of such reflection on reasoning occur in different fields...
, and
episodic memoryEpisodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...
. The study allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial cognitive tests while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.
Memory seems to be affected differently by certain stages of sleep such as REM and
slow-wave sleepSlow-wave sleep , often referred to as deep sleep, consists of stages three and four of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3...
(SWS). In one study cited in Born, Rasch, and Gais, multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and were then tested on it, both on early and late nights, with the order of nights balanced across participants. When the subjects' brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night, representing around 23% on average for sleep stage activity. The early-night test group performed 16% better on the
declarative memoryDeclarative memory is the aspect of human memory that stores facts. It is so called because it refers to memories that can be consciously discussed, or declared. It applies to standard textbook learning and knowledge, as well as memories that can be 'travelled back to' in one's 'mind's eye'. It is...
test than the control group. During late-night sleep, REM became the most active sleep stage at about 24%, and the late-night test group performed 25% better on the
procedural memoryProcedural memory is the long-term memory of skills and procedures, or "how to" knowledge .It is considered a form of implicit memory.-Process:...
test than the control group. This indicates that procedural memory benefits from late, REM-rich sleep, whereas declarative memory benefits from early, SWS-rich sleep.
A study conducted by Datta indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed wherein a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a five-second delay, an electrical shock would be applied. Once the shock commenced, the rat could move to the other end of the box, ending the shock immediately. The rat could also use the five-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded five seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial, but the rats were shocked regardless of their reaction. After each of the training sessions, the rat would be placed in a recording cage for six hours of polygraphic recordings. This process was repeated for three consecutive days. This study found that during the posttrial sleep recording session, rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and
procedural knowledgeProcedural knowledge is the knowledge exercised in the performance of some task. See below for the specific meaning of this term in cognitive psychology and intellectual property law....
.
An observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimoti, Barnes, and McNaughton. This study shows that after spatial exploration activity, patterns of
hippocampalThe hippocampus is a major component of the brains of humans and other mammals. It belongs to the limbic system and plays important roles in long-term memory and spatial navigation. Like the cerebral cortex, with which it is closely associated, it is a paired structure, with mirror-image halves in...
place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al., seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward-based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods, EEG data were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during prebehavior SWS (PRE) and three ten-minute intervals in postbehavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that ten minutes after the trial RUN session, there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level; however, after 20 minutes, the mean firing rate returned rapidly toward the PRE level. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta's study, as it also dealt with a form of spatial exploration.
The different studies all suggest that there is a correlation between sleep and the complex functions of memory. Harvard sleep researchers Saper and Stickgold point out that an essential part of memory and learning consists of nerve cell
dendriteDendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...
s' sending information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and they suggest that this may be why it is during sleep that we solidify memories and organize knowledge.
Preservation
The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the animal during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way, where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats. (Allison & Cicchetti, 1976; Webb, 1982).
However, this theory fails to explain why the brain disengages from the external environment during normal sleep. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive"; animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions.
Moreover, the preservation theory does not explain why carnivores like lions, which are on top of the
food chainFood chains describe the eating relationships between species within an ecosystem or a particular living place. Many types of food chains or webs are applicable depending on habitat or environmental factors...
, sleep the most. By the preservation logic, these top carnivores should not need any sleep at all. Preservation also does not explain why aquatic mammals sleep while moving. Lethargy during these vulnerable hours would do the same and would be more advantageous, because the animal will be quiescent but still be able to respond to environmental challenges like predators, etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but still occurs for a reason. For example, a zebra falling asleep the day after it spent the sleeping time running from a lion is more, not less, vulnerable to predation.
Dreaming
Dreaming is the perception of sensory images and sounds during sleep, in a sequence which the dreamer usually perceives more as an apparent participant than an observer. Dreaming is stimulated by the
ponsThe pons is a structure located on the brain stem. It is cranial to the medulla oblongata, caudal to the midbrain, and ventral to the cerebellum...
and mostly occurs during the REM phase of sleep.
People have proposed many
hypothesesA hypothesis is a proposed explanation for an observable phenomenon. The term derives from the Greek, ὑποτιθέναι - hypotithenai meaning "to put under" or "to suppose." For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
about the functions of dreaming.
Sigmund FreudSigmund Freud , Sigismund Schlomo Freud , was an Austrian neurologist who founded the psychoanalytic school of psychology...
postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the
unconscious mindThe unconscious mind is a term invented by the 18th century German romantic philosopher Ser Christopher Riegel and later introduced into English by the poet and essayist Samuel Taylor Coleridge...
, and he used
dream interpretationFor the John Cale minimalist album, see Dream Interpretation Dream interpretation is the process of assigning meaning to dreams. In many of the ancient societies, including Egypt and Greece, dreaming was considered a supernatural communication or a means of divine intervention, whose message could...
in the form of
psychoanalysisPsychoanalysis is a body of ideas developed by Austrian physician Sigmund Freud and continued by others. It is primarily devoted to the study of human psychological functioning and behavior, although it also can be applied to societies.
...
to uncover these desires. See Freud:
The Interpretation of DreamsThe Interpretation of Dreams is a book by psychoanalyst Sigmund Freud. The first edition was first published in German in November 1899 as Die Traumdeutung...
.
Freud's work concerns the psychological role of dreams, which clearly does not exclude any physiological role they may have. It is not ruled out therefore by the increased modern interest in the organization and consolidation of recent
memoryIn psychology, memory is an organism's mental ability to store, retain, and recall information. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing the memory....
and experience. Recent
research claims that sleep has this overall role of consolidation and organization of synaptic connections formed during learning and experience.
John Allan HobsonJohn Allan Hobson, M.D. is an American psychiatrist and dream researcher.He is known for his research on the Rapid eye movement sleep. He is Professor of Psychiatry, Emeritus, Harvard Medical School,...
and
Robert McCarley'sRobert W. McCarley, MD, is Chair and Professor of Psychiatry at Harvard Medical School and the VA Boston Healthcare System. He is also Director of the Laboratory of Neuroscience located at the Brockton VA Medical Center and the McLean Hospital...
activation synthesis theoryActivation-synthesis hypothesis is a neurobiological theory of dreams forwarded by Harvard University psychiatrists James Allan Hobson and Robert McCarley, first published on the American Journal of Psychiatry in December of 1977. It states that dreams are a random event caused by firing of neurons...
proposes that dreams are caused by the random firing of neurons in the
cerebral cortexThe cerebral cortex is a structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It constitutes the outermost layer of the cerebrum. In preserved brains, it has a grey color, hence the name "grey matter"...
during the REM period. According to this theory, the forebrain then creates a
storyA narrative is a story that is created in a constructive format that describes a sequence of fictional or non-fictional events...
in an attempt to reconcile and make sense of the nonsensical sensory information presented to it; hence, the odd nature of many dreams.
Sedatives
- Nonbenzodiazepine
The nonbenzodiazepines, also called benzodiazepine-like drugs, are a class of psychoactive drugs whose pharmacological actions are similar to those of the benzodiazepines, but are structurally distant or unrelated to the benzodiazepines on a chemical level...
hypnotics, better known as sleep aids, such as eszopicloneEszopiclone, marketed by Sepracor under the brand-name Lunesta, is a nonbenzodiazepine hypnotic agent used as a treatment for insomnia. Eszopiclone is the active stereoisomer of zopiclone, and belongs to the class of drugs known as cyclopyrrolones.Eszopiclone is a short acting nonbenzodiazepine...
(Lunesta), zalepon (Sonata)Zaleplon is a sedative/hypnotic, mainly used for insomnia. It is a nonbenzodiazepine hypnotic from the pyrazolopyrimidine class. In terms of adverse effects zaleplon appears to offer little improvement compared to both benzodiazepines and other non-benzodiazepine Z-drugs.Sonata is manufactured by...
, and zolpidem (Ambien)Zolpidem is a prescription medication used for the short-term treatment of insomnia, as well as some brain disorders. It is a short-acting nonbenzodiazepine hypnotic that potentiates gamma-aminobutyric acid , an inhibitory neurotransmitter, by binding to gamma-aminobutyric acid receptors at the...
are commonly used as sleep aids prescribed by doctors to treat forms of insomnia. Nonbenzodiazopines are the most commonly prescribed and OTCOTC may refer to:* Owatonna Tool Company* Oklahoma Tax Commission* Odenton Town Center* Officer in Tactical Command* Officer Training Corps* Offshore Technology Conference* Ohio Turnpike Commission* The Oliver Typewriter Company...
sleep aids used worldwide and have been greatly growing in use since the 1990s. They target the GABAA receptor.
- Benzodiazepine
A benzodiazepine is a psychoactive drug whose core chemical structure is the fusion of a benzene ring and a diazepine ring...
- Antihistamine
An H
1 antagonist is a histamine antagonist of the H
1 receptor that serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions...
Depressants
- Alcohol
An alcoholic beverage is a drink that contains ethanol . Alcoholic beverages are divided into three general classes: beers, wines, and spirits....
Often, people start drinking alcohol in order to get to sleep (alcohol is initially a sedative and will cause
somnolenceSomnolence is a state of near-sleep, a strong desire for sleep, or sleeping for unusually long periods . It has two distinct meanings, referring both to the usual state preceding falling asleep, and the chronic condition referring to being in that state independent of a circadian rhythm...
, encouraging sleep). However, being addicted to alcohol can lead to disrupted sleep, because alcohol has a
rebound effectRebound effect is the tendency of a medication, when discontinued, to cause a return of the symptoms being treated to be more severe than before...
later in the night. As a result, there is strong evidence linking alcoholism and insomnia.
- Barbiturate
Barbiturates are drugs that act as central nervous system depressants, and, by virtue of this, they produce a wide spectrum of effects, from mild sedation to total anesthesia. They are also effective as anxiolytics, hypnotics and as anticonvulsants. They have addiction potential, both physical and...
s
Barbiturates cause drowsiness and have actions similar to alcohol in that it has a
rebound effectRebound effect is the tendency of a medication, when discontinued, to cause a return of the symptoms being treated to be more severe than before...
and inhibits REM sleep, so it is not used as a long term sleep aid.
- Melatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring hormone found in animals and in some other living organisms, including algae. Circulating levels vary in a daily cycle, and melatonin is important in the regulation of the circadian rhythms of several...
Melatonin is a naturally occurring hormone that regulates sleepiness. It is made in the brain, where tryptophan is converted into serotonin and then into melatonin, which is released at night by the
pineal glandThe pineal gland is a small endocrine gland in the vertebrate brain. It produces melatonin, a hormone that affects the modulation of wake/sleep patterns and photoperiodic functions...
to induce and maintain sleep. Melatonin supplementation may be used as a sleep aid, both as a
hypnoticHypnotic drugs are a class of psychoactives whose primary function is to induce sleep and to be used in the treatment of insomnia and in surgical anesthesia. Because drugs in this class generally produce dose-dependent effects, ranging from anxiolysis to production of unconsciousness, they are...
and as a chronobiotic (see
phase response curveA phase response curve illustrates the relationship between the timing and the effect of a treatment designed to affect circadian rhythms. Normally, the various rhythms will be in synchrony within an individual , and sleep-wake is the most obvious of these rhythms...
, PRC).
- Siesta
A siesta is a short nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those where the weather is warm...
and the "post-lunch dip"
Many people have a temporary drop in alertness in the early afternoon, commonly known as the "post-lunch dip." While a large meal can make a person feel sleepy, the post-lunch dip is mostly an effect of the
biological clockA circadian rhythm is a roughly-24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria...
. People naturally feel most sleepy (have the greatest "drive for sleep") at two times of the day about 12 hours apart—for example, at 2:00 a.m. and 2:00 p.m. At those two times, the body clock "kicks in." At about 2 p.m. (14:00), it overrides the homeostatic buildup of sleep debt, allowing several more hours of wakefulness. At about 2 a.m. (02:00), with the daily sleep debt paid off, it "kicks in" again to ensure a few more hours of sleep.
- Tryptophan
Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...
The amino acid tryptophan is a building block of proteins. It has been claimed to contribute to sleepiness, since it is a precursor of the neurotransmitter serotonin, involved in sleep regulation. However, no solid data have ever linked modest dietary changes in tryptophan to changes in sleep.
Stimulants
Amphetamines (
amphetamineAmphetamine is a psychostimulant drug that is known to produce increased wakefulness and focus in association with decreased fatigue and appetite. Amphetamine is related to drugs such as methamphetamine and dextroamphetamine, which are a group of potent drugs that act by increasing levels of...
,
dextroamphetamineDextroamphetamine is a psychostimulant drug which is known to produce increased wakefulness and focus in association with decreased fatigue and decreased appetite. Drugs with similar psychoactive properties can be referred to or described as "amphetamine analogues", "amphetamine-like", or having...
,
methamphetamineMethamphetamine also known as metamfetamine , dextromethamphetamine, methylamphetamine, N-methylamphetamine, and desoxyephedrine) is a psychostimulant and sympathomimetic drug. Methamphetamine enters the brain and triggers a cascading release of dopamine, serotonin and norepinephrine...
, etc.) are often used to treat
narcolepsyNarcolepsy is chronic sleep disorder, or dyssomnia. The condition is characterized by excessive daytime sleepiness in which a person experiences extreme fatigue and possibly falls asleep at inappropriate times, such as while at work or at school...
and ADHD disorders and when used recreationally may be referred to as "speed." Their most common effects are decreased hunger, anxiety, insomnia, stimulation, and increased alertness.
AdderallAdderall is a brand-name psychostimulant medication composed of amphetamine and dextroamphetamine, which is thought to work by increasing the amount of dopamine and norepinephrine in the brain. Adderall is widely reported to increase alertness, libido, concentration and overall cognitive...
is a mixture of amphetamine salts used to treat ADHD.
- Caffeine
Caffeine is a bitter, white crystalline xanthine alkaloid that is a psychoactive stimulant drug. Caffeine was discovered by a German chemist, Friedrich Ferdinand Runge, in 1819. He coined the term kaffein, a chemical compound in coffee, which in English became caffeine...
Caffeine is a
stimulantStimulants, also sometimes called psychostimulants, are psychoactive drugs which induce temporary improvements in either mental or physical function or both. Examples of these kinds of effects may include enhanced alertness, wakefulness, and locomotion, among others...
that works by slowing the action of the hormones in the brain that cause
somnolenceSomnolence is a state of near-sleep, a strong desire for sleep, or sleeping for unusually long periods . It has two distinct meanings, referring both to the usual state preceding falling asleep, and the chronic condition referring to being in that state independent of a circadian rhythm...
, particularly by acting as an
antagonistAn antagonist is a character, group of characters, or an institution, who represents the opposition against which the protagonist must contend...
at
adenosineAdenosine is a nucleoside composed of a molecule of adenine attached to a ribose sugar molecule moiety via a β-N
9-glycosidic bond....
receptors. Effective dosage is individual, in part dependent on prior usage. It can cause a rapid reduction in alertness as it wears off.
- Cocaine
Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system and an appetite suppressant...
and crack cocaineCrack cocaine, crack or rock is a solid, smokable form of cocaine. It is a freebase form of cocaine that can be made using baking soda or sodium hydroxide, in a process to convert cocaine hydrochloride into methylbenzoylecgonine .-Appearance and characteristics:Crack cocaine as sold on the...
Studies on cocaine have shown its effects to be mediated through the circadian rhythm system. This may be related to the onset of
hypersomniaHypersomnia is a disorder characterized by excessive amounts of sleepiness.From the website of the U.S. National Institute of Neurological Disorders and Stroke :-Causes:...
(oversleeping) in regard to "Cocaine-Induced Sleep Disorder."
The stimulating effects of energy drinks come from stimulants such as
caffeineCaffeine is a bitter, white crystalline xanthine alkaloid that is a psychoactive stimulant drug. Caffeine was discovered by a German chemist, Friedrich Ferdinand Runge, in 1819. He coined the term kaffein, a chemical compound in coffee, which in English became caffeine...
, sugars, and essential amino acids, and they will eventually create a rapid reduction in alertness similar to that of caffeine.
- MDMA
MDMA is a psychoactive amphetamine drug with entactogenic, psychedelic, and stimulant effects....
, including similar drugs like MDA3,4-Methylenedioxyamphetamine is a psychedelic, stimulant, and empathogen-entactogen of the phenethylamine and amphetamine chemical classes...
, MMDA, or bk-MDMAMethylone , also known as 3,4-methylenedioxy-N-methylcathinone , is a psychoactive drug of the phenethylamine, amphetamine, and cathinone chemical classes that acts as an entactogen, psychedelic, and stimulant...
The class of drugs called
empathogen-entactogenThe terms empathogen and entactogen are used to describe a class of psychoactive drugs that produce distinctive emotional and social effects similar to those of MDMA . Putative members of this class include MDMA, MDA, MDEA, MBDB, and AET, among others...
s keep users awake with intense euphoria. Commonly known as "ecstasy."
- Methylphenidate
Methylphenidate is the most commonly prescribed psychostimulant and is indicated in the treatment of attention-deficit hyperactivity disorder, Postural Orthostatic Tachycardia Syndrome and narcolepsy, although off-label uses include treating lethargy, depression, neural insult, and obesity.In...
Commonly known by the brand names
Ritalin and
Concerta, methylphenidate is similar in action to amphetamines and cocaine.
Causes of difficulty in sleeping
There are many reasons for poor sleep. Following
sleep hygienicSleep hygiene can be defined as controlling "all behavioural and environmental factors that precede sleep and may interfere with sleep." It is the practice of following guidelines, usually simple and sensible ones, in an attempt to ensure more restful, effective sleep which can promote daytime...
principles may solve problems of physical or emotional discomfort. When pain, illness, drugs, or stress are the culprit, the cause must be treated.
Sleep disorderA sleep disorder is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning...
s (including the
sleep apneaSleep apnea is a sleep disorder characterized by pauses in breathing during sleep. Each episode, called an apnea , lasts long enough so that one or more breaths are missed, and such episodes occur repeatedly throughout sleep...
s,
narcolepsyNarcolepsy is chronic sleep disorder, or dyssomnia. The condition is characterized by excessive daytime sleepiness in which a person experiences extreme fatigue and possibly falls asleep at inappropriate times, such as while at work or at school...
, primary
insomniaInsomnia is a symptom of any of several sleep disorders, characterized by persistent difficulty falling asleep or staying asleep despite the opportunity. Insomnia is a symptom, not a stand-alone diagnosis or a disease. By definition, insomnia is "difficulty initiating or maintaining sleep, or both"...
,
periodic limb movement disorderPeriodic Limb Movement Disorder , previously known as Nocturnal myoclonus, is a sleep disorder where the patient moves limbs involuntarily during sleep, and has symptoms or problems related to the movement....
(PLMD), restless leg syndrome (RLS), and the
circadian rhythm sleep disorderCircadian rhythm sleep disorders are a family of sleep disorders affecting, among other things, the timing of sleep. People with circadian rhythm sleep disorders are unable to sleep and wake at the times required for normal work, school, and social needs. They are generally able to get enough sleep...
s) are treatable.
Older peopleOld age consists of ages nearing or surpassing the average life span of human beings, and thus the end of the human life cycle. Euphemisms and terms for old people include seniors , Senior Citizens , or the elderly...
are more easily awoken by disturbances in the environment and may to some degree lose the ability to consolidate sleep. They need the same amount per day as they've always needed, but may need to take some of their sleep as daytime naps.
Anthropology of sleep
Research suggests that sleep patterns vary significantly across cultures. The most striking differences are between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that prelight cultures have more broken-up sleep patterns. For example, people might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours. The boundaries between sleeping and waking are blurred in these societies. Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterised primarily by deep sleep and the second by REM sleep. This
segmented sleepSegmented sleep, divided sleep, bimodal sleep pattern and interrupted sleep are modern Western terms for a polyphasic or biphasic sleep pattern found in medieval and early modern Europe and many non-industrialised societies today, where the night's sleep is divided by one or more periods of...
has led to expressions such as "first sleep," "watch," and "second sleep," which appear in literature from preindustrial societies all over the world.
Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many nomadic or
hunter-gathererA hunter-gatherer society is one whose primary subsistence method involves the direct procurement of edible plants and animals from the wild, foraging and hunting without significant recourse to the domestication of either...
societies, people will sleep on and off throughout the day or night depending on what is happening.
Plentiful artificial light has been available in the industrialised West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.
In some societies, people generally sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone but a most intimate relation, such as a spouse. In almost all societies, sleeping partners are strongly regulated by social standards. For example, people might only sleep with their immediate family,
extended familyThe term Extended family has several distinct meanings. First, it is used synonymously with consanguineous family or joint family. Second, in societies dominated by the conjugal family or nuclear family, it is used to refer to kindred who does not belong to the conjugal family. Often there could be...
, spouses, their children, children of a certain age, children of specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity.
People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or
bedA bed is a piece of furniture used as a place to sleep.Beds usually consist of a mattress placed on top of a box spring inner-sprung base...
s. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, and the incidence of pests.
Sleep in nonhumans
Many animals sleep, but neurological sleep states are difficult to define in lower-order animals. In these animals, sleep is defined using behavioral characteristics such as minimal movement, postures typical for the species, and reduced responsiveness to external stimulation. Sleep is quickly reversible, as opposed to hibernation or
comaIn medicine, a coma is a profound state of unconsciousness. A comatose person cannot be awakened, fails to respond normally to pain or light, does not have sleep-wake cycles, and does not take voluntary actions....
, and sleep deprivation is followed by longer or deeper sleep. Herbivores, who require a long waking period to gather and consume their diet, typically sleep less each day than similarly sized carnivores, who might well consume several days' supply of meat in a sitting.
Horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Some aquatic mammals and some birds can sleep with one half of the brain while the other half is awake, so-called
unihemispheric slow-wave sleepUnihemispheric slow-wave sleep is sleep in which one half of an animal's brain is at rest, while the other half remains alert. During USWS, only one eye is closed, allowing the animal to remain alert to activity in its environment. It has been observed in various species of birds, dolphins,...
. Birds and mammals have cycles of non-REM and REM sleep (as described above for humans), though birds’ cycles are much shorter and they do not lose muscle tone (go limp) to the extent that most mammals do.
Many mammals sleep for a large proportion of each 24-hour period when they are very young. However, killer whales and some
dolphinDolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly...
s do not sleep during the first month of life. Such differences may be explained by the ability of land-mammal newborns to be easily protected by parents while sleeping, while marine animals must, even while very young, be more continuously vigilant for predators.
See also
- Alarm clock
An alarm clock is a clock that is designed to make a loud sound at a specific time. The primary use of these clocks is to awaken people from their sleep in order to start their days in the mornings, but can also be used for short naps; they are sometimes used for other reminders as well...
- Dream world (plot device)
Dream world is a commonly used plot device in fictional works, most notably in science fiction and fantasy fiction. The use of a dream world creates a situation whereby a character is placed in a marvellous and unpredictable environment and must overcome several personal problems to leave it...
- Microsleep
A microsleep is an episode of sleep which may last for a fraction of a second or up to thirty seconds. Often, it is the result of sleep deprivation, mental fatigue, sleep apnea, hypoxia, narcolepsy, or hypersomnia...
- Morvan's syndrome
Morvan’s Syndrome, or Morvan’s fibrillary chorea , is a rare autoimmune disease named after nineteenth century French physician Augustin Marie Morvan. “La chorée fibrillaire” was first coined by Morvan in 1890 when describing patients with multiple, irregular contractions of the long muscles,...
- Sudden infant death syndrome
Sudden infant death syndrome or crib death is a syndrome marked by the sudden death of an infant that is unexpected by history and remains unexplained after a thorough forensic autopsy and a detailed death scene investigation...
- Cortisol awakening response
The cortisol awakening response is an increase of about 50% in cortisol levels occurring 20 to 30 minutes after awakening in the morning in some people. This rise is superimposed upon the late-night rise in cortisol which occurs before awakening...
Positions, practices, and rituals
- Co-sleeping
Co-sleeping, also called the family bed, is a practice in which babies and young children sleep with one or both parents, as opposed to a separate infant bed...
- Hypnosis
Hypnosis is a mental state or set of attitudes usually induced by a procedure known as a hypnotic induction, which is commonly composed of a series of preliminary instructions and suggestions. Hypnotic suggestions may be delivered by a hypnotist in the presence of the subject, or may be...
- Meditation
Meditation is used here as a broad term for practices done by a sole practitioner without much, if any, external aide, often for the purpose of self-transformation...
- Neutral spine
A neutral spine or good posture refers to the "three natural curves [that] are present in a healthy spine."-Posture:The word "posture" comes from the Latin verb "ponere" which is defined as "to put or place." The general concept of human posture refers to "the carriage of the body as a whole, the...
- Sleep hygiene
Sleep hygiene can be defined as controlling "all behavioural and environmental factors that precede sleep and may interfere with sleep." It is the practice of following guidelines, usually simple and sensible ones, in an attempt to ensure more restful, effective sleep which can promote daytime...
- Yoga Nidra
Yoga Nidra, may be rendered in English as "yogic sleep" or "sleep of the yogis". There are numerous traditions of Yoga Nidra sadhana that have been transmitted through parampara within the Indian religions. These aspects may include disciplines and traditions of dream, sleep and yoga...
- Siesta
A siesta is a short nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those where the weather is warm...
External links
- Healthy Sleep from the Division of Sleep Medicine at Harvard Medical School
Harvard Medical School is one of the graduate schools of Harvard University. It is currently ranked first among American research medical schools by U.S. News and World Report....
and WGBHWGBH is a non-commercial television and radio broadcast service located in Boston, Massachusetts. WGBH is a member station of the Public Broadcasting Service, and has produced many programs for the network, including nearly a third of PBS's national primetime programming...
Educational Foundation
- Is Sleep Essential? by Chiara Cirelli and Giulio Tononi, from the Public Library of Science
The Public Library of Science is a nonprofit open-access scientific publishing project aimed at creating a library of open access journals and other scientific literature under an open content license. It launched its first journal, PLoS Biology, in October 2003 and has steadily created another...
Biology
- National Center on Sleep Disorders Research