Orexin

Orexin-A and orexin-B, known equally as hypocretin-1 and hypocretin-2, are cleaved from a single precursor made by a small population of neurons in the lateral hypothalamus. Those neurons project widely to the monoaminergic and cholinergic arousal nuclei and act through two G protein-coupled receptors: OX1R, which binds orexin-A preferentially, and OX2R, which binds both. Their function is less to generate wakefulness than to stabilise it, holding the sleep-wake switch in one state and preventing rapid flipping between them. They also contribute to feeding, reward and autonomic tone.

The clinical anchor is narcolepsy type 1, in which the orexin neurons are destroyed, most likely by an autoimmune mechanism, and cerebrospinal fluid orexin-A falls to very low or undetectable levels. That measurement is a diagnostic criterion. The pathway was identified in 1998 by two independent groups, and shortly after, canine narcolepsy was traced to a mutation in the OX2R gene. Pharmacology followed in both directions: dual orexin receptor antagonists including suvorexant, lemborexant and daridorexant are approved for insomnia and scheduled as controlled substances, while orexin receptor agonists are in clinical development for narcolepsy after an earlier candidate was discontinued over liver safety signals.

What the system offers is a way to promote sleep by removing a wake signal rather than by amplifying GABA-A inhibition, which changes the side-effect profile and the effect on sleep architecture. It is not a general sedative mechanism and does not substitute for treating the cause of insomnia.

The misuse to expect is orexin invoked as an appetite or energy lever in consumer marketing. Naming the receptor family does not make an unstudied compound an orexin ligand, and the approved antagonists are prescription controlled drugs, not sleep aids of the kind sold alongside them.