For 50 years, serotonin has been in the centre of the search for the mechanisms and control of sleep. Serotonergic neurotransmission is related to the behavioural state of the animal and plays an important role in modulation of the behavioural state, by interacting with other brain areas modulating circadian rhythm, sleep and waking. Serotonergic activity may be accompanied by waking or sleep depending on the brain area and receptor type involved in the response, on the current behavioural state and on the concomitant agonism/antagonism of other neurotransmitter systems.