What is it? In this condition, the person's biological clock is 25 hours or longer, meaning that sleep and wake times are continually getting later.
How common is it? While this condition is rare, it seems to be more common in the blind than in the general population. One study showed that 75% of blind people complain of sleep problems. This may, in part, be due to the lack of light cues for waking and sleeping.
What's the treatment? Light therapy, in which the person is exposed to bright lights (as bright as sunlight) for two hours in the morning and avoids bright light for several hours before bedtime to shift the sleep-wake cycle, can be effective in patients with sight.