Orexin, also called hypocretin, is the common name given to a pair of hormones simultaneously discovered by two groups of reseachers in rat brains. Masashi Yanagisawa and colleagues at a Howard Hughes Medical Institute-funded lab at the University of Texas, coined the term orexin to reflect the orexigenic (appetite-stimulating) activity of these hormones. DeLecea, Killduff, and colleagues also reported discovery of these same hormones, dubbing them hypocretins to indicate that they are synthesized in the hypothalamus and to reflect their similarity to a class of hormones called incretins (i.e., hypothalamic incretin).

Orexin has been found to have great importance in the study of appetite and sleep, specifically narcolepsy. When an animal wakes up orexin is distributed throughout the brain from the deep hypothalamus. When an animal goes to sleep the levels of orexin fade and leave the brain. Humans with little or no orexin are narcoleptic; they fall asleep unintentionally and uncontrollably.

The research on orexin is still in an early phase, although many scientists believe that orexin-based drugs could help narcoleptics and increase alertness in the brain without the side effects of amphetamines.

The discovery of the Orexin hormone pair calls into question the ancient assumption that wakefulness is the natural state of animal life. It strongly suggests that the natural state of animal life is sleep, a constructive, anabolic process, and that the only biological significance of wakefulness, a destructive, catabolic process, is its value as an activity during which the animal organism ingests nutrients and reproduces, similar to the way that single celled animals enter vibrant, albeit temporary, periods of heightened activity to ingest nutrients and/or to undergo cell division - the primordial progenitive activity.