Non-24-hour sleep–wake disorder

Non-24-hour sleep–wake disorder (non-24 or N24SWD) is one of several chronic circadian rhythm sleep disorders (CRSDs). It is defined as a 'chronic steady pattern comprising daily delays in sleep onset and wake times in an individual living in society'. Symptoms result when the non-entrained (free-running) endogenous circadian rhythm drifts out of alignment with the light/dark cycle in nature. Although this sleep disorder is more common in blind people, affecting up to 70% of the totally blind, it can also affect sighted people. Non-24 may also be comorbid with bipolar disorder, depression, and traumatic brain injury. The American Academy of Sleep Medicine (AASM) has provided CRSD guidelines since 2007 with the latest update released in 2015. In people with non-24, the body essentially insists that the length of a day (and night) is appreciably longer (or, very rarely, shorter) than 24 hours and refuses to adjust to the external light–dark cycle. This makes it impossible to sleep at normal times and also causes daily shifts in other aspects of the circadian rhythm such as peak time of alertness, body temperature minimum, metabolism and hormone secretion. Non-24-hour sleep–wake disorder causes a person's sleep–wake cycle to move around the clock every day, to a degree dependent on the length of the cycle, 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 lower sleep propensity after total sleep deprivation than do normal sleepers. Non-24 can begin at any age, not uncommonly in childhood. It is sometimes preceded by delayed sleep phase disorder. Most people with this disorder find that it severely impairs their ability to function in school, in employment, and in their social lives. Typically, they are 'partially or totally unable to function in scheduled activities on a daily basis, and most cannot work at conventional jobs'. 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, to the point of falling into microsleeps, as well as myriad effects associated with acute and chronic sleep deprivation. Sighted people with non-24 who force themselves to live to 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.' It has been estimated that non-24 occurs in more than half of all people who are totally blind. The disorder can occur at any age, from birth onwards. It generally follows shortly after loss or removal of a person’s eyes, as the photosensitive ganglion cells in the retina are also removed. 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 non-24 for many totally blind individuals. Non-24 is rare among visually impaired patients who retain at least some light perception. Researchers have found that even minimal light exposure at night can affect the body clock. People with this disorder might find it difficult to follow a regular clock scheme, as their biological clock can shift so much that they are sleepy during the day and experience insomnia during night. Another common diagnostic feature is the cyclical nature of non-24: people will experience certain periods, whether they be weeks or months, of sleeping during the day (symptomatic periods) that alternate with periods of sleeping during normal nighttime hours (asymptomatic periods).