Mice with targeted deletion of the prepro-orexin gene (orexin−/− mice) display a phenotype strikingly similar to narcolepsy: abrupt behavioral arrests with muscle atonia (cataplexy), fragmented wakefulness (inability to maintain consolidated wakefulness episodes), and direct transitions from wakefulness to REM sleep (Chemelli et al., 1999). Here, HCRT is linked to narcolepsy.