Free-running sleep
Encyclopedia
Free-running sleep experiments can involve any organism which sleep
s. Free-running sleep is sleep which is not adjusted, entrained
, to the 24-hour cycle in nature nor to any artificial cycle. Such experiments are used in the study of circadian and other rhythms in biology
. Subjects are shielded from all time cues, often by a constant light protocol
, by a constant dark protocol or by the use of light/dark conditions to which the organism cannot entrain such as the ultrashort protocol of one hour dark and two hours light. Also, limited amounts of food can be made available at short intervals so as to avoid entrainment to mealtimes. Subjects are thus forced to live by their internal circadian "clocks".
The individual's or animal's circadian phase can be known only by the monitoring of some kind of output of the circadian system, the internal "body clock". The researcher can precisely determine, for example, the daily cycles of gene
-activity, body temperature, blood pressure
, hormone
secretion and/or sleep
and activity/alertness
. Alertness in humans can be determined by many kinds of verbal and non-verbal tests; activity in animals by observation, for example of wheel-running in rodent
s.
When animals or people free-run, experiments can be done to see what sort of signals, known as zeitgeber
s, are effective in entrainment. Also, much work has been done to see how long or short a circadian cycle the different organisms can be entrained to. For example, some animals can be entrained to a 22-hour day, but they can not be entrained to a 20-hour day. In recent studies funded by the U.S. space industry, it has been shown that most humans can be entrained to a 23.5-hour day and to a 24.65-hour day.
The effect of unintended time cues is called masking. If morning rush traffic can be heard from outside, if researchers or maintenance staff appear at the same time each day, an experiment can be ruined by masking.
s in humans. It affects more than half of people who are totally blind
(clinically known as NLP, no light perception) and a small number of sighted individuals.
Among blind people, the cause is the inability to register, and therefore to entrain to, light cues. The many blind people who do entrain to the 24-hour light/dark cycle have eyes with functioning retinas including operative non-visual light-sensitive cells. These ganglion cell
s, which contain melanopsin
, convey their signals to the "circadian clock" via the retinohypothalamic tract
(distinct from the optic nerve
), linking the retina to the pineal gland
.
Among sighted individuals, FRD usually first appears in the teens or early twenties. As with delayed sleep phase disorder (DSPS or DSPD), in the absence of neurological damage due to trauma or stroke
, cases almost never appear after the age of 30. FRD affects many more sighted males than sighted females. A quarter of sighted individuals with FRD also have an associated psychiatric condition, and a quarter of them have previously shown symptoms of DSPS.
The term free-running sleep has occasionally been used by non-scientists to indicate intentional facilitation of the natural sleep/wake cycle. In this context, free-running sleep means that a person chooses to sleep when sleepy and to awaken spontaneously (specifically without an alarm clock
or reference to the time of day). A decision to prioritize a natural sleep schedule over all other schedules can create conflicts with employment and social obligations.
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...
s. Free-running sleep is sleep which is not adjusted, entrained
Entrainment (chronobiology)
Entrainment, within the study of chronobiology, occurs when rhythmic physiological or behavioral events match their period and phase to that of an environmental oscillation. A common example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by...
, to the 24-hour cycle in nature nor to any artificial cycle. Such experiments are used in the study of circadian and other rhythms in biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
. Subjects are shielded from all time cues, often by a constant light protocol
Protocol (natural sciences)
In the natural sciences a protocol is a predefined written procedural method in the design and implementation of experiments. Protocols are written whenever it is desirable to standardize a laboratory method to ensure successful replication of results by others in the same laboratory or by other...
, by a constant dark protocol or by the use of light/dark conditions to which the organism cannot entrain such as the ultrashort protocol of one hour dark and two hours light. Also, limited amounts of food can be made available at short intervals so as to avoid entrainment to mealtimes. Subjects are thus forced to live by their internal circadian "clocks".
The individual's or animal's circadian phase can be known only by the monitoring of some kind of output of the circadian system, the internal "body clock". The researcher can precisely determine, for example, the daily cycles of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
-activity, body temperature, blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
, hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
secretion and/or sleep
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...
and activity/alertness
Alertness
Alertness is the state of paying close and continuous attention, being watchful and prompt to meet danger or emergency, or being quick to perceive and act. It is related to psychology as well as to physiology...
. Alertness in humans can be determined by many kinds of verbal and non-verbal tests; activity in animals by observation, for example of wheel-running in rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s.
When animals or people free-run, experiments can be done to see what sort of signals, known as zeitgeber
Zeitgeber
Zeitgeber is any exogenous cue that synchronizes an organism's endogenous time-keeping system to the earth's 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light...
s, are effective in entrainment. Also, much work has been done to see how long or short a circadian cycle the different organisms can be entrained to. For example, some animals can be entrained to a 22-hour day, but they can not be entrained to a 20-hour day. In recent studies funded by the U.S. space industry, it has been shown that most humans can be entrained to a 23.5-hour day and to a 24.65-hour day.
The effect of unintended time cues is called masking. If morning rush traffic can be heard from outside, if researchers or maintenance staff appear at the same time each day, an experiment can be ruined by masking.
In humans
Non-24-hour sleep-wake disorder, also referred to as free-running disorder (FRD) or Non-24, is one of the circadian rhythm sleep disorderCircadian rhythm sleep disorder
Circadian 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 in humans. It affects more than half of people who are totally blind
Blindness
Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define blindness...
(clinically known as NLP, no light perception) and a small number of sighted individuals.
Among blind people, the cause is the inability to register, and therefore to entrain to, light cues. The many blind people who do entrain to the 24-hour light/dark cycle have eyes with functioning retinas including operative non-visual light-sensitive cells. These ganglion cell
Ganglion cell
A retinal ganglion cell is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells...
s, which contain melanopsin
Melanopsin
Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...
, convey their signals to the "circadian clock" via the retinohypothalamic tract
Retinohypothalamic tract
The retinohypothalamic tract is a photic input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells , which contain the photopigment melanopsin...
(distinct from the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
), linking the retina to the pineal gland
Pineal gland
The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...
.
Among sighted individuals, FRD usually first appears in the teens or early twenties. As with delayed sleep phase disorder (DSPS or DSPD), in the absence of neurological damage due to trauma or stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
, cases almost never appear after the age of 30. FRD affects many more sighted males than sighted females. A quarter of sighted individuals with FRD also have an associated psychiatric condition, and a quarter of them have previously shown symptoms of DSPS.
The term free-running sleep has occasionally been used by non-scientists to indicate intentional facilitation of the natural sleep/wake cycle. In this context, free-running sleep means that a person chooses to sleep when sleepy and to awaken spontaneously (specifically without an alarm clock
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 night's sleep or short naps; they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock is pressed; but...
or reference to the time of day). A decision to prioritize a natural sleep schedule over all other schedules can create conflicts with employment and social obligations.
External links
- A collection of articles about sleep by Piotr A. Wozniak, July 2000