sleep

Sleep is the state of natural rest observed in humans and throughout the animal kingdom, including in all mammals and birds, and in many reptiles, amphibians and fish.[1] It is characterized by a reduction in voluntary body movement, temporary blindness, decreased reaction to external stimuli, loss of consciousness, an increased rate of anabolism (the synthesis of cell structures), and a decreased rate of catabolism (the breakdown of cell structures). In humans, other mammals and many other animals which have been studied, such as fish, birds, mice, ants and fruitflies, regular sleep is necessary for survival. The capability for arousal from sleep is a protective mechanism and also necessary for health and survival.