Sighted vs. Blind
Although the majority of people with Non 24-Hour Sleep Wake Disorder (Non-24) are blind, it also occurs in a small number of sighted individuals. As many as half to three-quarters of totally blind patients (i.e., having no light perception) are considered to have Non-24, representing approximately 65 to 95 thousand Americans. The number of sighted people with Non-24 is unknown.
Most blind people have some light perception. As a result, their circadian rhythms are synchronized to a 24-hour day-night cycle, as in the sighted. However, it has been estimated that of the 1.3 million blind people in the United States, 10% have no light perception. These totally blind individuals are at the greatest risk for Non-24.
Non-24 is common in totally blind people due to a lack of light entering the eyes. Photoreceptors in the retina normally signal the brain and regulate the 24-hour day-night cycle. For a totally blind individual with Non-24, their inability to perceive light prevents the synchronization of their internal body clock to the day-night cycle. As a result, the internal body clock defaults to a non-24 hour cycle, causing fluctuating periods of good sleep followed by periods of poor sleep and excessive daytime sleepiness.
The exact cause of Non-24 in sighted people is unknown. But, it is believed to be caused by neurological factors.