Orexin

Orexin is a neuropeptide (a molecule used by neurons for communication) involved in sleep, appetite, and arousal functions. Orexin is involved in hunger and appetite by increasing the desire for food. Evidence shows a relationship between orexin and the hunger hormones ghrelin and leptin (which are appetite stimulators and suppressors respectively). Orexin is involved in wakefulness and determines whether or not an organism should be asleep or awake.

A lack of orexin is involved with the incidence of narcolepsy