And maybe a new mattress, because it’s not just the amount of time you spend sleeping that keeps you lean, it’s also the quality of your sleep. Fat cells in your body produce a hormone called leptin that helps the body keep track of how much potential energy (i.e. fat) it has stored. But leptin is only produced during certain stages of sleep. Miss out on those stages because you’re not resting soundly enough, and you’ll disturb levels of the hormone, leaving your body with no real idea of its energy reserves. Consequently, you’ll end up storing calories rather than burning them.