orexin

Orexin, also known as hypocretin, is a chemical messenger produced by a small population of nerve cells in the hypothalamus, a region deep in the brain that helps regulate sleep. The National Institute of Neurological Disorders and Stroke describes it as a natural chemical that helps a person stay awake and that controls rapid eye movement sleep, the stage in which most dreaming happens. The cells that make it are few in number but their reach is wide, and they act to stabilise the states of sleep and waking so that the boundary between the two stays distinct.

The clearest evidence of what orexin does comes from what happens when it is missing. Nearly all people with narcolepsy type 1 have extremely low levels of it, and the loss of the orexin-producing neurons in the hypothalamus is the main cause of that condition. Without those cells the brain struggles to hold a steady waking state, and features of REM sleep, including the muscle limpness that normally keeps a sleeper from acting out a dream, can intrude while the person is awake. That is why the loss produces not one symptom but a cluster of them: daytime sleepiness, cataplexy, sleep paralysis, hallucinations at the edge of sleep, and broken nights.

The reason for the cell loss is not fully understood. Current research points to an immune attack on the orexin-containing cells, arising from a combination of genetic and environmental factors, with a variation in the gene HLA-DQB1 raising the chance of developing the disorder. Because the deficiency itself is the root of the disease, replacing the missing signal has been a long-standing goal of drug development, and medicines that activate the orexin receptor directly reached regulatory approval for the first time in 2026.