• A natural and periodic state of rest during which consciousness of the world is suspended.
  NCI
  U.S. National Cancer Institute, 2021
• A readily reversible suspension of sensorimotor interaction with the environment, usually associated with recumbency and immobility.
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2025
• Any process in which an organism enters and maintains a periodic, readily reversible state of reduced awareness and metabolic activity. Usually accompanied by physical relaxation, the onset of sleep in humans and other mammals is marked by a change in the electrical activity of the brain.
  Gene Ontology Dictionary
  Gene Ontology Consortium, 2021
• Period of rest for the body and mind during which volition and consciousness are in partial or complete abeyance and bodily functions are partially suspended; a behavioral state marked by a characteristic immobile posture and diminished but readily reversible sensitivity to external stimuli.
  CRISP Thesaurus
  National Institutes of Health, 2006
• Sleep is necessary for survival and good health, but why sleep is needed and exactly how it benefits people are not fully understood. One of sleep's benefits is its restorative effect on people's ability to function normally during the daytime.
  Merck Manuals
  Merck & Co., Inc., 2025
• Sleep, a normal, reversible, recurrent state of reduced responsiveness to external stimulation that is accompanied by complex and predictable changes in physiology. These changes include coordinated, spontaneous, and internally generated brain activity as well as fluctuations in hormone levels and...
  Encyclopedia Britannica
  Encyclopedia Britannica, Inc., 2025