Sleep is defined as a reversible state of immobility with a significant reduction in response to environmental stimuli. Criteria used for determinating sleep in an observed animal are behavioral, electrophysiological, physiological and homeostatic. Sleep cycles are composed of REM and NREM sleep. NREM sleep is characterised by minimal mental activity , while typical characteristics of REM sleep include general muscle atonia, rapid eye movements nad vivid dreaming. The ascending reticular activating system is composed of two branches – cholinergic and monoaminergic. Cholinergic neurons are active in waking and REM states, while the activity of monoaminergic neurons decreases in NREM and ceases in REM sleep. The suprachiasmatic nucleus maintains wakefulness during the light phase of the day, while the ventrolateral preoptic nucleus inhibits the ascending reticular activating system during the night. NREM and REM sleep have been confirmed only in birds and mammals and their origin in this two classes is thought to be convergent. There is no consensus whether REM and NREM sleep are also present in other vertebrate classes. Among vertebrates who never sleep are troglobionts, sharks and teleosts. Their common characteristic ist that they have a significantly decreased need for processing of visual stimuli. The link between decreased need for processing of sensory stimuli and absence of sleep lies in Rauscheker's fundamental dogma of neurosciences, which states that there is a conflict between processing of sensory information and refreshing of longterm memories. Sleep evolved because of a need to refresh long-term memories without the interference of environmental stimuli. The hypothesis of synaptic homeostasis explains the role of slow-wave sleep in maintance of number and strenght of synapses, whereby it allows for brain plasticity.