[The current concepts of the nature and functions of sleep].

The paper presents a review of current papers concerning the functional significance of sleep in the whole and of its phases. A new hypothesis is proposed according to which the main sleep functions are body energy conservation and saving of plastic substances. The author reasons that the healthy human brain and the animal brain all the more do not become tired under low and moderate mental and physical loading in the day-time and, therefore, do not need sleep as rest.