Light wakes people up. Darkness means it’s time to sleep. That cycle helps keep the circadian rhythm, or body clock, in sync with the world. This is important because left to its own devices, the body clock is actually set to a 24 hour and 11 minute day—which is a factor in seasonal affective disorder and the reason jet lag is generally more severe traveling east (which shortens the day) than west (which lengthens it).
Some people have what are called circadian rhythm disorders, in which the biological rhythm does not properly synchronize with the rest of the person’s life. These are largely culturally and socially defined disorders—the patient’s sleep and other functioning are perfectly normal, just not at appropriate times; someone whose lifestyle does not require them to sleep, work, or eat according to a specific schedule may be perfectly fine with what would otherwise be deemed a circadian rhythm disorder. However, that doesn’t make these conditions any less real, nor any less of a problem for people who do suffer from them.
One of the most severe of these disorders is called non-24-hour sleep-wake disorder, or Non-24. Patients with Non-24 have a body clock that is significantly longer than 24 hours and 11 minutes—sometimes as long as 26 hours—and that doesn’t reset in response to light cues. Often this is because the person doesn’t get those cues; the condition affects as many as 80 percent of blind people, though only two percent of the population as a whole—but there is also a neurological component that can prevent the brain from using these cues.
Treatment involves light therapy and administration of the sleep hormone melatonin, but these are not effective in blind patients who cannot perceive light—there is no approved treatment for these patients. Not everyone with Non-24 needs or seeks treatment; sometimes it can be productive. The novelist George Dawes Green has used his sleepless nights to write several successful novels, and an insomniac storytelling session was the seed of the Moth podcast and performance series.