Non-24: A Debilitating Circadian Rhythm Disorder

Many people have occasional trouble sleeping. Chronic circadian rhythm disorders are much more serious—and can disrupt the ability to adhere to a typical 24-hour schedule.

Non-24-Hour Disorder (Non-24) is a serious, rare, and chronic circadian rhythm disorder that affects 70% of people who are totally blind and lack light perception. Non-24 is common in people who are totally blind because without light perception, the master body clock is unable to sync with the 24-hour day-night cycle. Overall, about 65,000-95,000 people in the United States are estimated to have Non-24.

Most people have a master body clock that naturally runs longer than 24 hours, and light is the primary environmental cue that resets it to 24 hours each day. People with Non-24 have a master body clock that continually delays, putting them to sleep later and later each day, turning night into day and day into night, until the cycle starts all over again. People who are blind and living with Non-24 report that Non-24 can be highly disruptive, making it difficult to do well in school, hold down a job, or maintain relationships.

The disorder can occur at any age, and usually coincides with or follows shortly after loss of light perception, or loss or surgical removal of the eyes. The causes of blindness do not seem to be related to the risk of developing the disorder.

Without the ability to see light, the brain is not cued to reset the master body clock, which guides many of the body’s functions, including day-night cycles, body temperature, and the release of hormones, to a 24-hour cycle. So the day may run slightly longer than 24 hours, causing a delay in the body clock each day.

As the master body clock becomes out of sync with the 24-hour day-night cycle, people with Non-24 begin to have trouble going to sleep on a regular schedule. As the day-night cycle moves gradually later and later—the average circadian period is 24.5 hours—sleeping at night becomes more and more difficult, and increases the drive to sleep during the day.

Eventually, the day-night cycle moves back into sync with the night, and people with Non-24 can sleep well during what is considered to be a typical sleep period. But this is only temporary; the misalignment between the internal circadian rhythm and the 24-hour day-night cycle starts all over again, and again the sleep cycle begins to shift. This dyssynchrony, when the cycle is out of sync, may last for about one to four months before the master body clock resets and the day-night cycle is temporarily restored.

Non-24 can also affect people who are not blind. However, these cases are rare, and the true rate of the disorder in the general population is not known.