We spend nearly one-third of our lives asleep and some rodents even spend half or more experience in this state.However,the neuronal mechanism of sleep is still one of the great unsolved scientific questions.One hypothesis is that sleep results from the inhibition of wake-promoting systems.The rostromedial tegmental nucleus(RMTg),a newly identified structure in the brainstem,is rich in μ-opioid receptors and primarily comprises GABAergic neurons.