A natural hormone, leptin alerts the brain that the body has sufficient fatty tissue. Study: sleep deprivation is associated with lower levels of leptin, a signal to the brain that more calories were needed. This hormonal imbalance triggered by lack of sleep could favor weight gain. (Gorman, Christine. Why We Sleep. p 46-59. NY:Time Inc., Time. Vol 164, No 25, Dec 20, 2004.)