Throughout life human beings maintain an alternating rhythm of two biological rhythms between three states: wakefulness, NREM sleep and REM sleep. These states are quite distinct both with regard to conduct and physiologically. Sleep is an active process during which an amalgam of behavioural and physiological processes occur, together with complex cerebral activity. The universality of sleep or similar resting behaviour in the animal kingdom has encouraged study of its origin and function. The phylogenetics of sleep and the study of behaviour during this state in different species of animals offers ample possibilities to understand its significance. We describe the characteristics of conduct which define the different phases of sleep in adult humans. Motor activity, relationship with the surroundings, perception of and reaction to external stimuli, ability to recognize and remember are all variables which are modified during the transition from wakefulness to sleep, NREM sleep and REM sleep. Changes in behaviour during sleep form a group of pathological conditions with different origins and physiopathological methods involved. The parasomnias are the alterations of behaviour during sleep par excellence. We describe the concept and classification according to current theories.