Orexin (hypocretin). In the late 1990s, researchers discovered orexin (hypocretin), a neurotransmitter involved in the brain’s pleasure and reward system. In animal studies, researchers could initiate or reduce cravings for drugs and food by activating or blocking orexin (G. C. Harris et al., 2005). Orexin is also involved in regulating sleep and wakefulness. For instance, low levels of this neurotransmitter have been linked to a serious sleep disorder called narcolepsy