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How many hours of sleep is enough?  Most people need from seven to nine hours of sleep a day. Most people sleep seven – eight hours a day and experience mental and physical impairment if they are deprived of such a chance. However, for some of us three – four hours are enough.  Most scientists suggest that varying amount of necessary sleep for different people accounts for genetic mechanisms. In comparison with nineteenth century the duration of the sleep has been 20% reduced. Now we sleep half and an hour less than two centuries ago. Technological innovations appear to break the natural rhythm of our body, thus affecting our biological watch.