The mesopontine tegmentum has a pivotal role for regulation of paradoxical sleep. Noradrenergic neurons in the locus coeruleus and serotonergic neurons in the dorsal raphe have similar firing properties during sleep-waking cycles (active in waking and silent during paradoxical sleep). However, several findings suggest that these two populations of neurons have different influences on sleep-waking regulation. A population of cholinergic neurons in the brainstem which are most active during paradoxical sleep (PS-on neuron) are inhibited by carbachol and excited by bicuculline, while those of another population, active both during waking and paradoxical sleep, are inhibited by carbachol and excited by noradrenaline and histamine. Non-cholinergic PS-on neurons are excited by carbachol and inhibited by noradrenaline. Slow wave sleep is thought to be generated by a neural mechanism in the preoptic area, but neurons in the preoptic area display several kinds of firing patterns in relation with sleep-waking cycles.