According to the experiments, many blind people

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? (14) It turns out that many blind people do have the same sleep cycle as sighted people. How can this happen? The answer is; hormones—one hormone called melatonin in particular. (15) 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. (16) 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. However, this doesn’t work for all blind people. Further research may be able to explain this sensitivity to light in terms of the type of blindness of the subject.