Stages of sleep evolution (facts and hypotheses).

The paper contains new data concerning the peculiarities of sleep in poikilotherms (fish, amphibians, reptiles). A conclusion is drawn that sleep and the activation spontaneously arising against its background are the universal phenomena in vertebrates. A supposition is argued about the existence of evolutionary succession between the forms of sleep inherent in lower vertebrates and the quiet and active phases of homoiotherms' sleep.