<unk> , also termed circadian rhythm sleep disorder, free-running type, circadian arrhythmia, non-24-hour circadian rhythm disorder or hypernychthemeral<unk> , is a circadian rhythm sleep disorder. The International Classification of Sleep Disorders defines the disorder as, "a chronic steady pattern comprising one- to two-hour daily delays in sleep onset and<unk>  times in an individual living in society."
In people with this disorder, the person's body essentially insists that the day is longer than<unk> <unk> s. This tends to not allow socially accepted sleeping patterns, and makes it difficult for the sufferer to sleep at "normal" times. Most cases that have been reported in the medical literature have occurred in blind patients.
Left untreated,<unk>  causes a person's sleep-wake cycle to change every day, the degree determined by how much over<unk> <unk> s 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<unk> s in different seasons, and growth spurts, which are typically known to cause fatigue.
Common treatments for<unk>  are similar to those for delayed sleep phase<unk> . 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<unk> s 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.
Non-24-hour sleep-wake syndrome
Circadian rhythm sleep disorder
Delayed sleep phase syndrome
Sleep hyperhidrosis
Advanced sleep phase syndrome
Non-rapid eye movement sleep arousal disorder
Insomnia
Shift work sleep disorder