Sleep EEG was recorded and analyzed in patients with neurotic insomnia. It was found that interhemispheric asymmetry in the same individual can vary during sleep from right-hemispheric to left-hemispheric. Interhemispheric EEG asymmetry is closely related to the stage of sleep. The development of left-hemisphere or right-hemisphere asymmetry is mainly determined by activity of the right hemisphere. The development of interhemispheric asymmetry during wakefulness, stages 1 and 2 sleep, and delta sleep is mediated by common mechanisms.