In animals, sleep is a naturally recurring state characterized by altered consciousness, relatively inhibited sensory activity, and inhibition of nearly all voluntary muscles. It is distinguished from wakefulness by a decreased ability to react to stimuli, and it is more easily reversed via stimuli than the state of hibernation or of being comatose.During sleep, most systems in an animal are in a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal, and muscular systems. Sleep in non-human animals is observed in mammals, birds, reptiles, amphibians, and fish, and in some form in insects and even in simpler animals such as nematodes, suggesting that sleep is universal in the animal kingdom.The purposes and mechanisms of sleep are only partially clear and the subject of substantial ongoing research. Sleep is sometimes thought to help conserve energy, though this theory is not fully adequate as it only decreases metabolism by about 5–10%. Additionally it is observed that mammals require sleep even during the hypometabolic state of hibernation, in which circumstance it is actually a net loss of energy as the animal returns from hypothermia to euthermia in order to sleep.Humans may suffer from a number of sleep disorders. These include dyssomnias (such as insomnia, hypersomnia, and sleep apnea), parasomnias (such as sleepwalking and REM behavior disorder), and the circadian rhythm sleep disorders.