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Research has shown the connection between short sleep duration and elevated levels of ghrelin, which is commonly known as the “hunger hormone.” Levels of ghrelin are high and you have bigger appetite, Malkoff-Cohen says. That’s why increased Body Mass Index (BMI) has been observed in people who didn’t sleep enough, she adds. How much is enough varies with every person but experts say a good night’s sleep lasts between seven and eight hours.