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 disorders 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 cells, 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 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.
Read more about this topic: Free-running Sleep
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