For most adults, the circadian rhythms that guide the sleep-wake cycle reset approximately every 24 hours. This is why many people start to get sleepy around the same time each night. In contrast, people with non-24-hour sleep wake disorder have circadian rhythms that are either shorter or longer than 24 hours. Affected individuals progressively shift their sleep and wake times one to two hours earlier or later each day.
Individuals with non-24-hour sleep-wake disorder have a circadian rhythm that is shorter or, more often, slightly longer than 24 hours. This causes sleep and wake times to get pushed progressively earlier or later, usually by one or two hours at a time. Over days or weeks, the circadian rhythm becomes desynchronized from regular daylight hours.
As a consequence of this ever-shifting rhythm, individuals with N24SWD experience inappropriate fluctuations in appetite, mood, and alertness. During periods when their body clock is heavily desynchronized, they show a natural preference for sleeping in the middle of the day and difficulty sleeping at night. Several weeks later, they may not show any symptoms at all as their internal clock catches up with daylight once more.
Attempts to maintain a regular sleep-wake cycle are unsuccessful, even when supplemented by common solutions such as caffeine. Over the long term, desynchronization from the innate circadian rhythm may have adverse health consequences.
Individuals with non-24-hour sleep-wake disorder often have difficulty keeping work, school, or social commitments. They may develop depression due to the stress of not being able to keep to a normal schedule, or as a side effect of sleeping during the day and not getting enough sunlight.
Non-24-hour sleep-wake disorder is one of six circadian rhythm sleep. It is considered an intrinsic sleep-wake disorder because the problem is mostly due to internal factors instead of external factors, such as jet lag or shift work.
Other intrinsic circadian rhythm disorders include advanced and delayed sleep-wake phase disorders, in which the sleep-wake cycle is pushed significantly forward or backward, and irregular sleep-wake rhythm disorder, in which individuals display a fragmented sleep-wake cycle with napping throughout the day and long periods awake at night.
Non-24-hour sleep-wake disorder is most common in people with total, due to the lack of light input reaching the internal clock. It’s estimated that approximately 50% of completely blind people have N24SWD. Not all blind people suffer from this disorder, because some retain the ability to perceive light to a certain extent.
For many people with N24SWD, the circadian rhythm drifts faster when bedtime occurs during the day and slows down when bedtime coincides with night. Researchers hypothesize that blind people may display a weak circadian response to factors such as sleep schedules, physical activity, and possibly even light, but not a strong enough response to establish a regular circadian rhythm.
Most people with N24SWD have between a 24 and a 25-hour sleep cycle. The further the cycle is from 24 hours, the faster disruptions to their sleep-wake schedule occur.
Non-24-hour sleep-wake disorder can also happen in sighted people. Symptoms manifest as generic daytime sleepiness and nighttime insomnia, so N24SWD is often misdiagnosed as another sleep disorder in sighted people. As a result, many sighted people have the disorder for years before receiving a diagnosis.
It’s unclear what causes non-24-hour sleep-wake disorder in sighted people. The largest study ever done on sighted people with N24SWD found that the majority were male, with symptoms that started in the teens or twenties. Non-24-hour sleep-wake disorder may have a genetic component, though it rarely runs in families and may not develop unless a person has more than one risk factor.
A significant percentage of sighted people with N24SWD also have a prior diagnosis of a mental health disorder such as major depression, bipolar disorder, obsessive-compulsive disorder, and schizophrenia or schizoid personality. For these individuals, N24SWD may have developed as a result of social isolation and other side effects of their disorder. Researchers suspect that N24SWD may sometimes develop naturally in people as a side effect of staying up late for many years and getting too much nighttime exposure to light.
In some cases, non-24-hour sleep-wake disorder in sighted people may be linked to a traumatic brain injury. Damage to cells in the retina, to the pathway linking the retina and the SCN, to the pathway that regulates melatonin secretion, or to the SCN itself may disrupt or weaken the body clock.