Orexin A and B, or hypocretin-1 and -2) are two related peptides, with approximately 50% sequence identity, are produced by cleavage of a single precursor protein.  Orexins, also called hypocretins, are the common names given to a pair of highly excitatory neuropeptide hormones that were simultaneously discovered by two groups of researchers in rat brains.

Studies suggest that orexin A/hypocretin-1 may be of greater biological importance than orexin B/hypocretin-2. Although these peptides are produced by a very small population of cells in the lateral and posterior hypothalamus, they send projections throughout the brain.  The orexins/hypocretins are strongly conserved peptides, found in all major classes of vertebrates. The peptides are thought to have arisen early in vertebrate evolution.