Sleep images
In animals, sleep is a naturally recurring state characterized by altered consciousness, relatively inhibited sensory activity, and inhibition of nearly all voluntary muscles.[1] It is distinguished from wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than being in hibernation or a coma.
During sleep, most systems in an animal are in a heightened anabolic state, accentuating the growth and rejuvenation of, e.g., the immune, nervous, skeletal and muscular systems. It is observed in mammals, birds, reptiles, amphibians and fish, and in some form also in insects and even simpler animals such as nematodes (see the related article Sleep (non-human)), suggesting that sleep is universal in the animal kingdom.
