Fat?

The effects on little sleep, like six hours or less, can impact your appetite hormones, like leptin, which regulates your weight and appetite and signals to the brain of how much energy is in the body, and grehlin which causes the desire to eat. Also, since there are more waking hours for people who sleep less, they tend to eat more. Another reason is if you feel groggy, your rationale about food intake goes out the window.