Non-24-Hour Sleep–Wake Syndrome

Non-24-hour sleep–wake syndrome, sometimes shortened to 'non-24', is one of several types of so-called chronic circadian rhythm sleep disorders (CRSDs). The problem occurs when your non-entrained (free-running) endogenous circadian rhythm drifts out of alignment with the desired or conventional sleep–wake schedule, which is normally the 24-hour light–dark cycle. Thus your internal clock is out of sync with the normal cycle of night (dark) and day (light) .

In the majority of cases the patient is blind and the syndrome is easily explained by an absence of light – called photic input – to the circadian clock. However, the disorder can also occur in people with normal eyesight for reasons that are not well understood but it remains an extremely rare syndrome, with fewer than 100 cases of sighted people with non-24 reported in the scientific literature[1].

The disorder is an invisible disability that can be "extremely debilitating in that it is incompatible with most social and professional obligations".

People with the disorder may have an especially hard time adjusting to changes in 'normal' sleep–wake cycles, such as during 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. Disrupted circadian rhytm is associated with depression[2].

[1] Hayakawa et al (2005): Clinical analyses of sighted patients with non-24-hour sleep–wake syndrome: a study of 57 consecutively diagnosed cases in Sleep – 2005
[2] Mendlewicz: Disruption of the circadian timing systems: molecular mechanisms in mood disorders in CNS Drugs – 2009