Non-24 is a chronic circadian rhythm sleep disorder defined as "a chronic steady pattern comprising one-to-two-hour daily delays in sleep onset and wake times in an individual living in society." The pattern of delay persists "around the clock," taking several weeks to complete one cycle. This disruption of the body clock causes cyclical bouts of nighttime sleeplessness and excessive daytime fatigue and napping.

It is a neurological sleep disorder that has been traced to genetic abnormalities that affect specific factors in the brain. While anyone can be diagnosed with Non-24, the disorder affects proportionately more totally blind individuals than sighted.