A rare condition called non-24 sleep-wake disorder, or “non-24,” characterized by a body clock that is out of synch with the 24-hour cycle of the Earth day, can affect those with normal vision, but it especially plagues the totally blind who can’t perceive light. Of approximately 100,000 totally blind people in the United States, anywhere from 55 percent to 70 percent of them may suffer from non-24, according to Harvard neuroscientist Steven Lockley.
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