Sleeping is the widespread appearance in all orders from the subphylum vertebrate. Because of the numerous anatomical differences and unique habitats that they inhabit, the numerous ways of sleeping have evolved also. No matter how particular species sleeps, its function is generally common. The sleeping theories underline development of brain during sleeping period, avoiding predators, memory consolidation, energy conservation and protection against oxidative stress. However, it is sure that sleeping is a necessity in vertebrate’s physiology. Researches on different mammalian orders have confirmed that sleep deprivation drives to descent of vital functions and finally to death. Classical bilateral sleeping of mammals is divided into several phases characterized by different EEG waves of neocortex. Sleeping begins with NREM phase, in which alpha, theta, delta waves, sleep spindle and K complex are alternating during first four stages. Then comes the REM phase in which the PGO waves occur. Rapid eye movements, changes in breathing rhythm and pulse, finger twitching and erection in males are observed in this phase. Unilateral way of sleeping in marine mammals, Cetacea and Otariidae, is unique in vertebrate’s biology. Maintaining one hemisphere awake, they manage to monitor turbulent marine environment and to avoid possible predators. At the same time, in the other hemisphere slow-wave EEG activity characteristic for classical sleeping is dominant. Fur seals are specific because while in water they show the way of sleeping similar to that in Cetaceans, and when they are on land they transfer to bilateral sleeping. Further researches on this field will lead to new tremendous discoveries in biology of sleeping, which will help us to supplement our puzzle of knowledge about one of the most important biology phenomenon in general.