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Non-24-hour sleep-wake syndrome, also termed non-24-hour circadian rhythm disorder or hypernychthemeral syndrome, is a sleep disorder in which a person's internal clock runs longer than 24 hours. The person's body essentially insists that the day is longer than 24 hours, which may not allow socially accepted sleeping patterns and make it difficult for the sufferer to sleep at "normal times."

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 may go around the clock, eventually returning to "normal" for one or two days before going "off" again. This is known as "free-running".

People with the disorder often have a hard time "resetting" their internal clocks to socially accepted sleep-wake patterns. They 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.

Treatment may be sought. Common treatments for non-24-hour sleep-wake syndrome are similar to those for delayed sleep phase syndrome. They include light therapy with a full spectrum lamp giving—usually—10000 lux, chronotherapy, acupuncture, vitamin B12 supplements, and melatonin supplements. It often takes several treatments before any progress is noticed, and for many the treatments may only be marginally effective or not effective at all. In addition, the treatment is not a cure, and the condition can only be managed.

Treatment with melatonin taken 30 minutes to two hours before the desired bedtime may be helpful in righting a pattern "gone awry". Too high a dose of melatonin may have the unintended effect of disturbing the sleep or even causing nightmares, and uncontrollable yawning the next day. Bright light therapy combined with the use of melatonin may be the most effective treatment. The timing of both is tricky and a lot of determination and experimentation is usually necessary.


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