A definition of sleep would have to include the features of rhythmicity, reversibility, selective and variable suppression of responses to incoming stimuli, and certain somatic and psychic activities that proceed on a plane below the threshold of what is termed consciousness. Sleep is differentiated from stupor and coma by the rapidity and completeness of the return to wakefulness; from catatonia and catalepsy by the flaccidity and quick response to stimuli, and from syncope by the evanescence and unexpectedness, as well as by the circulatory phenomena, of the latter.
Sleep has been utilized in psychoanalysis not only to reveal the patient's hidden complexes in the shape of dreams, but also as an example of the wish to return to the maternal uterus. The curled up posture assumed by the normal sleeping individual is supposed to resemble the fetal attitude.
Sleep is also interpreted as a death-wish, as an instinct, as a