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Sighted
In sighted people with this disorder, the body essentially insists that the day is longer than 24 hours and refuses to adjust to the external light/dark cycle. This makes it very difficult, if not impossible, to sleep at conventional times. In addition to the sleep propensity, other circadian rhythms such as peak time of alertness, body temperature minimum and hormone secretion shift later on a daily basis. Left untreated, non-24-hour sleep-wake syndrome causes a person's sleep-wake cycle to change every day, the degree determined by how much over 24 hours the cycle lasts. The cycle progresses around the clock, eventually returning to "normal" for one or two days before "going off" again. This is known as free-running sleep.

People with the disorder may have an especially hard time adjusting to changes in "regular" sleep-wake cycles, such as vacations, stress, evening activities, time changes like daylight saving time, travel to different time zones, illness, medications (especially stimulants or sedatives), changes in daylight hours in different seasons, and growth spurts, which are typically known to cause fatigue. They also show lowered sleep propensity after total sleep deprivation.[13]

Most people with this disorder find that it severely impairs their ability to function socially and occupationally. Typically, they are "partially or totally unable to function in scheduled activities on a daily basis, and most are unable to work at conventional jobs".[1] Attempts to keep conventional hours by people with the disorder generally result in insomnia (which is not a normal feature of the disorder itself) and excessive sleepiness,[1] to the point of falling into microsleeps, as well as myriad effects associated with acute and chronic sleep deprivation. People with Non-24 who force themselves to live on a normal workday "are not often successful and may develop physical and psychological complaints during waking hours, i.e. sleepiness, fatigue, headache, decreased appetite, or depressed mood. Patients often have difficulty maintaining ordinary social lives, and some of them lose their jobs or fail to attend school."[13]

The impact of Non-24 on daily life is affected by the extent to which the body clock drifts. For instance, a person with a 25-hour body clock would have a 25-day cycle with two weeks of good sleep and two weeks of poor sleep. But a person with a 24.1-hour circadian rhythm would drift only six minutes per day and take 241 days, or eight months, to go around the clock. They may have four months of good sleep and four months of bad sleep, when the cycle would begin again.

Blind
Most cases of Non-24 that have been reported in the medical literature have occurred in blind patients. One study suggested that Non-24 occurs in more than half of all people who are totally blind.[4] It can occur at any age, from birth onwards. It generally follows shortly after loss or removal of a person’s eyes. [5]

The degree to which free-running endogenous circadian rhythms effect the sleep pattern in blind people is quite variable. Some describe it as the most burdensome aspect of being blind, while other seem to be able to maintain a normal sleep wake pattern. This relative tolerance for free-running endogenous rhythms suggests that the Non-24 is a rather different syndrome for sighted and blind.

Sighted
Sighted people with Non-24 do exist, but are much more rare and the etiology of their circadian disorder is less well understood.[2] As of 2005, there were fewer than 100 cases of sighted people with N24 reported in the scientific literature.[8] At least one case of a sighted person developing Non-24 was preceded by head injury;[9] another patient diagnosed with the disorder was later found to have a "large pituitary adenoma that involved the optic chiasma".[1] Thus the problem appears to be neurological. Specifically, it is thought to involve abnormal functioning of the suprachiasmatic nucleus of the hypothalamus.[10] Several other cases have been preceded by chronotherapy, a prescribed treatment for delayed sleep phase syndrome.[2][11]

There have been several experimental studies of sighted people with the disorder. McArthur et al. reported a patient who "appeared to be subsensitive to bright light" based on the light induced suppression of melatonin secretion.[12] On the other hand, one study found that they were more sensitive than the control group.[13]) In 2002 Uchiyama et al. examined five sighted Non-24 patients who showed, during the study, a sleep-wake cycle averaging 25.12 hours.[14] That is appreciably longer than the 24.02-hour average shown by the control subjects in that study, which was near the average innate cycle for healthy adults of all ages: the 24.18 hours found by Charles Czeisler.[15] The literature usually refers to a "one to two hour" delay per 24-hour day (i.e. a 25–26 hour cycle).

Uchiyama et al. had earlier determined that sighted Non-24 patients' minimum core body temperature occurs much earlier in the sleep episode than the normal two hours before awakening. They suggest that the long interval between the temperature trough and awakening makes illumination upon awakening virtually ineffective,[16] as per the phase response curve (PRC) for light.

In their Clinical Review in 2007, Okawa and Uchiyama reported that people with Non-24 have a mean habitual sleep duration of nine to ten hours and that their circadian periods average 24.8 hours.[13]

Blind
Without light to the retina, the suprachiasmatic nucleus (SCN), located in the hypothalamus, is not cued each day to synchronize the circadian rhythm to the 24-hour social day, resulting in free running circadian rhythms for a majority of totally blind individuals.[4] Non-24 is rare among visually impaired patients who retain at least some light perception. Researchers have found that even minimal light exposure can synchronize the body clock.[7]

Sighted
The first report and description of a case of Non-24, a man living on 26-hour days, was "A man with too long a day" by Ann L. Eliott et al. in November 1970.[17] The related and more common DSPS wasn't described until 1981.

Blind
In the first detailed study of Non-24 in a blind subject, researchers reported on a 28-year-old male who had a 24.9-hour rhythm in sleep, plasma cortisol, and other parameters. Even while adhering to a typical 24-hour schedule for bedtime, rise time, work, and meals, the man’s body rhythms continued to shift.[6]