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[Comment] Entraining the free-running circadian clocks of blind people

Most totally blind people with no perception of light suffer from non-24-hour sleep–wake disorder, also called free-running disorder. These people do not receive the light signals that ordinarily entrain (synchronise) their internal circadian clocks to the 24 h day. This causes recurring blocks of days or months with severe insomnia and excessive daytime sleepiness when the natural time for sleep occurs during the day instead of at night.1–3 Daily administration of appropriately timed melatonin has been shown to phase shift, and thus entrain, the circadian clock and is recommended by the Standards of Practice Committee of the American Academy of Sleep Medicine.