Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than being in hibernation or a coma. Sleep is a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.
The purposes and mechanisms of sleep are only partially clear and are the subject of intense research. Sleep is often thought to help conserve energy, but decreases metabolism only about 5–10%. Hibernating animals need to sleep despite the hypometabolism seen in hibernation, and must return from hypothermia to euthermia in order to sleep, making sleeping "energetically expensive."
Famous quotes containing the word sleep:
“To sleep around is absolutely wrong for a woman; its degrading and it completely ruins her personality. Sooner or later it will destroy all that is feminine and beautiful and idealistic in her.”
—Barbara Cartland (b. 1901)
“All sensuality is one, though it takes many forms; all purity is one. It is the same whether a man eat, or drink, or cohabit, or sleep sensually. They are but one appetite, and we only need to see a person do any one of these things to know how great a sensualist he is. The impure can neither stand nor sit with purity.”
—Henry David Thoreau (18171862)
“Was it for this the clay grew tall?
MO what made fatuous sunbeams toil
To break earths sleep at all?”
—Wilfred Owen (18931918)