Both leptin and ghrelin are hormones responsible for triggering hunger and appetite in a person's body. Studies have shown that when a person is sleep deprived, have reduced leptin levels and an elevated ghrelin levels.
Ghrelin is the hormone responsible for increasing appetite, when a person is sleep deprived the levels of ghrelin is elevated therefore encouraging the person to become hungry and as a result eat or overeat in the process.
Leptin is the hormone responsible for suppressing appetite and regulating ghrelin, when a person is sleep deprived the levels of leptin becomes reduced.
When such imbalance occurs, the body becomes less tolerant with leptin and more tolerant with ghrelin. In effect the body becomes less adaptive therefore it gains more weight because of the increase in appetite brought about by high levels of ghrelin,