From this research came the discovery that the orexin system is a critical regulator of sleep-wake cycles, with the loss of orexin neurons (Thannickal et al., 2000, 2009; Hara et al., 2001; Honda et al., 2009) and the resulting loss of endogenous orexin (Nishino et al., 2000, 2001; Peyron et al., 2000; Ripley et al., 2001; Kanbayashi et al., 2002; Mignot et al., 2002) responsible for the rapid cycling between sleep and wake states, both during day and night, characteristic of human narcolepsy. The gene discussed is HCRT; the disease is narcolepsy.