Causes And Risk Factors Of Non-24-Hour Sleep-Wake Disorder

Blindness

Most individuals affected by non-24-hour sleep-wake disorder are also completely blind. This complication occurs because the body's internal clock functions according to the way light is seen. As many as three-quarters of all blind individuals suffer from effects of non-24-hour sleep-wake disorder. The human eye has two key functions. The first is to see, and the other is to absorb light. When the eye takes in light, the time of day is signaled to the brain. Individuals who are blind have no light perception regardless of the cause of their blindness. In a healthy individual, the master body clock runs twenty-four hours a day and is reset each day by light from the environment that enters the eyes. Once entering the eyes, the light hits photosensitive cells that transmit signals to the individual's brain. Because this mechanism is not activated in blind individuals, their internal body clock cycle may last less than or more than twenty-four hours. The natural cycle of non-24-hour sleep-wake disorder causes a week of severe insomnia followed by three weeks of normal sleep. Because of such cycle, blind individuals with non-24-hour sleep-wake disorder may not know they are affected.

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