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A three-year study of over 20,000 people found that, compared to individuals who slept seven hours, those who slept less than five were more likely to experience weight gain and become obese. There are probably various reasons for this but Knutson says one culprit may be hormones that control our appetite. "Studies have seen that if you sleep restrict somebody, they do tend to eat more, even though everything else is controlled for," she says.