The three diffuse projection systems arising in the brainstem, that is, noradrenergic projection originating in the locus coeruleus, serotonergic projection from the dorsal raphe nucleus, and cholinergic projection from neurons gathering in the laterodorsal tegmental nucleus and scattering in the pedunculopontine tegmental nucleus, may function as controllers of sleep and wakefulness. We have investigated the functional roles of these projections by recording neuronal activity in these brainstem nuclei and by stimulating the brainstem nuclei. It is suggested that the projection from the locus coeruleus is an arousal system; the function of the serotonergic projection is still mysterious, since these neurons are active specifically during waking; activation of the noradrenergic projection excites the upper brain sites whereas activation of the serotonergic projection depressed them. It is clear that a group of cholinergic neurons constitute a system to induce and maintain paradoxical sleep. The cholinergic projection may also have the role to induce a rapid, transient elevation of the vigilance level by its phasic response to novel, unfamiliar stimuli.