The tuberomammillary nucleus (TMN) and ventrolateral preoptic nucleus (VLPO) form a bistable sleep/wake switch. Orexin (Ox) release from the perifornical (PF) area stabilizes the VLPO–TMN sleep switch and plays a key role in modulating outputs from several areas. Ach, acetylcholine; DA, dopamine; His, histamine; GABA, γ-aminobutyric acid; LC, locus coeruleus; NMB, nucleus basalis of Meynert; nor, norepinephrine; VTA; ventral tegmental area.