Hormones are substances in the body that help to regulate body processes. Over recent years, researchers have been learning a lot about the hormones that help to regulate hunger. Two of these hormones are called leptin and ghrelin. Leptin decreases hunger, so people do not feel hungry when leptin levels are high. Ghrelin increases hunger, so people feel hungry when ghrelin levels are high. Sleep plays an important role in the body's use and storage of energy from food. Researchers have found that lack of sleep leads to hunger in rodents and, possibly, hunger in humans. It is unknown whether lack of sleep influences the hormones that help to regulate hunger.