The melanin-concentrating hormone (MCH) is a 19 aminoacid peptide found in mammals predominantly in neurons located in the lateral hypothalamus and incerto-hypothalamic area. The biological function of MCH is mediated by two G-protein-coupled receptors known as MCHR1 and MCHR2, although the latter is expressed only in carnivores, primates and man. The MCHR1 couples to G_i, G_q and G_o proteins, with G_i leading to the inhibition of both excitatory and inhibitory synaptic events. Within the central nervous system (CNS) MCH participates in a number of functions including sleep–wake behavior. In this respect, MCHergic neurons project widely throughout the CNS to brain regions involved in the regulation of behavioral states. MCHergic neurons are silent during wakefulness (W), increase their firing during slow wave sleep (SWS) and still more during REM sleep (REMS). Studies in knockout mice for MCH (MCH^−/−) have shown a reduction in SWS and an increase of W during the light and the dark phase of the light-dark cycle. Moreover, in response to food deprivation a marked reduction in REMS time was observed in these animals. Conflicting effects on sleep variables have been reported in MCHR1^−/− mice by different authors. The i.c.v. administration of MCH increases REMS and SWS in the rat. In addition, an enhancement of REMS has been described following the microinjection of the neuropeptide into the nucleus pontis oralis of the cat, while its infusion into the dorsal raphe nucleus (DR) and the basal forebrain (horizontal limb of the diagonal band of Broca) is followed by an increase of REMS and a reduction of W in the rat. Immunoneutralization of MCH in the DR augmented W and suppressed REMS in the rat, as did the s.c. injection of selective MCHR1 antagonists. The robust REMS-inducing effect of MCH is likely related to the deactivation of monoaminergic, orexinergic, glutamatergic, cholinergic (W-on) and GABAergic (REM-off) neurons involved in the generation of W and the inhibition of REMS. On the basis of preclinical studies, it can be proposed that selective MCHR1 receptor agonists could constitute potential therapeutic modalities in the arsenal of insomnia pharmacotherapy. Due to the lack of adequate animal models, the role of the MCHR2 on sleep is still unknown.