An attempt was made to broadly consider energy exchange in the sleep—wake fulness cycle of cells not only in the brain, but also in other tissues of the body. For this purpose, the results of experiments on energy supply of tissues in sleep models performed on animals were generalized with the data of polysomnography and mass spectrometry of exhaled air of subjects examined during the night. Such an integrated approach allowed us to clarify the mechanisms of sleep, as well as to come closer to understanding its disorders in humans and to outline ways of their effective pharmacotherapy.