Free-running sleep is a sleep pattern characterised by sleep times not entrained to normal time cues; often referring to a later shifting (delay) on a daily basis. This occurs as the circadian rhythm sleep disorder non-24-hour sleep–wake disorder, or when artificially induced in experiments studying circadian rhythms and other biological rhythms within the field of chronobiology.
Background
An individual's or organism's circadian phase can be determined by monitoring an output of the circadian system, the internal circadian clock. A researcher can precisely determine factors such as daily cycles of genetic activity, body temperature, blood pressure, hormone secretion, or sleep and activity/alertness patterns. Alertness can be tracked in humans through performance-based tests or other measures (such as somnolence), whereas in animals it can be assessed by observing trends in physical activity (for example, wheel-running in rodents).
In research settings
When humans or organisms free-run, experiments can be conducted to determine which signals, known as zeitgebers, are effective in entrainment. Other rhythm disruptions may be used, such as providing limited amounts at short intervals to avoid entrainment to average mealtimes. These modifications can show the strength and influence of the intrinsic circadian clock.
Researchers can conduct studies to measure entrainment for various organisms and the bounds of circadian cycle lengths. For example, some animals can be entrained to a 22-hour day, but not to a 20-hour day. Observations of free-running and circadian rhythm flexibility are also applicable to situations such as sleep during space travel, for example, for reduced day lengths required of astronauts over short periods, or when considering the length of day on a planet such as Mars (24.65 hours). The difference between circadian length and the 24-hour day, while inadequately synchronised by zeitgebers, results in free running. Non-24 affects more than half of people who are totally blind and a small number of sighted individuals.
Among blind people, the cause is the inability to adequately register light cues, preventing entrainment. These ganglion cells, which contain melanopsin, relay signals to the circadian clock via the retinohypothalamic tract (branching off from the optic nerve), which is the pathway from the retina to the pineal gland.
Among sighted individuals, non-24 usually first appears during 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 rarely develop after age 30.