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 which is a sleep disorder that can cause chronic tiredness, falling asleep at inappropriate times, and cataplexy. Research has also shown a relationship between orexin and mood with higher levels associated with feelings of happiness and lower levels associated with sadness. Orexin was only discovered in the 1990s so much more research needs to be conducted in order to find the full scope of this neuropeptide’s influence and function.