Orexin (Hypocretin)

Orexin (also called hypocretin) is a small signaling molecule, a peptide, made by a tiny cluster of cells deep in the brain in a region called the hypothalamus. Think of it as the hand that holds the light switch of wakefulness firmly in the ON position. When orexin is being released, you stay clearly, steadily awake; when its grip loosens, the switch starts flickering and you drift toward sleep.

These orexin cells reach out to many of the brain's other arousal systems and keep them all switched on together, so that being awake feels like one smooth, continuous state instead of a series of stop-and-start moments. Its real importance becomes obvious when it is lost: in the sleep disorder narcolepsy, most of the orexin-making cells die off, and without that steady hand the boundary between waking and sleeping collapses. People feel overwhelmingly sleepy during the day, can fall asleep without warning, and may suddenly go limp with strong emotion (a feature called cataplexy), as if a piece of dreaming sleep has leaked into waking life.