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Did you ever wonder why it is that most people are "programmed" to sleep at night instead of during the day? If there's something about the cycle of light and dark that's telling us when to sleep, then shouldn't the sleep cycle of a blind person be different? As it turns out, many blind people—people with no visual perception of light at all—do have the same sleep cycle as sighted people. So now you're wondering, "How can this happen?" The answer is: hormones, one hormone in particular. It's called melatonin. In sighted people, the level of melatonin goes up at night, or when it's dark, and goes down in the day, or when it's light. It's believed that it's the presence of this hormone in the blood that gives us the urge to sleep. If an increase in melatonin level "programs" sighted people to sleep at night, then what about blind people? A researcher, named Dr. Charles Czeisler, tells about an interesting experiment. He tried shining a bright light into the eyes of some blind people. When he did this, he noticed that the level of the melatonin in the blood of these subjects went down just as it would do for sighted people. Somehow, the eyes of these subjects, even though they were damaged and had no visual perception of light, could tell their brain when there was more or less light. Now, this doesn't work for all blind people; in fact, most of Czeisler's subjects had no hormonal response to light at all. Further research may be able to explain this sensitivity to light in terms of the type of blindness of the subject.