how much sleep do you really need?

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I can't seem to get enough sleep lately, for a variety of reason I won't discuss here. My doctors and all books I read say that you need a good amount of sleep, 7-8 hours. But I remember reading in some magazines that some successful people got very little sleep. so where is the truth? is 5 hours enough sleep? or do you really need 7-8 for a healthy life?