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Sleep is a mystery.

Scientists acknowledge certain objective features of sleep, but otherwise differ in what they see as its essential elements. In defining it, would they emphasize the biological nature of the activity, its purpose or its benefits? Would "sleep" encompass those squishy states of semi-consciousness between being asleep and awake? What about alcohol black-outs and comas, or when you zone out during slide 51 of a coworker's scintillating presentation on growth tactics? And we haven't even broached the philosophical side of things. 

So we asked a variety of experts to answer the question "What is sleep?" in one sentence or less. Some definitions are specific to certain functions; others are wonderfully vague. All of them offer perspective on the universal act of rest.  


1. "Sleep is the nightlife of the brain that functions to sustain and nourish our waking lives."

— John Peever, Professor Professor of Cell and Systems Biology, University of Toronto

2. "Sleep is a fascinating scientific mystery — and a great personal blessing." 

— Hank Greely, Professor of Law and Genetics, Stanford University

3. "The promise that never stops promising."

—RM Vaughan, Essayist

4. "Having your brain almost completely disconnected from the outside world but still living an internal life singular to you, while your body rests, lying down."

— Célyne Bastien, Psychologist, Laval University, Québec, Canada

"The promise that never stops promising."

5. "Sleep is a physiological state that allows the restoration of physical and mental health processes and access into different psychological states."

— Flavie Waters, Research Professor, University of Western Australia  

6. "When our animal bodies slip into the womb of healing and forgetting, our souls re-awaken and remember." 

— Ryan Hurd, Founder of'

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7. "Sleep consists of two distinct states of consciousness, and it is as vital to animal life as oxygen, food and water." 

Sean P.A. Drummond, Professor of Clinical Neuroscience, Monash University

8. "A natural state of rest when the cells of the body can regenerate." 

— Debra Jaliman, Author of “Skin Rules: Trade Secrets from a Top New York Dermatologist"

9. "Sleep is critical for maintaining the optimal health of every organ in your body, and sleep loss is a physiological stress that can damage even the DNA in the cells throughout your brain and body." 

 — Bryce Mander, postdoctoral Psychology fellow, UC Berkeley as vital to animal life as oxygen

10. "A state of greatly reduced activity and responsiveness that is homeostatically regulated." 

— Jerome Siegel, Director of Siegel Lab, Psychiatry and Biobehavioral Sciences, Center for Sleep Research, UCLA

11. "Characterized by a blunted ability to respond to external stimuli, response to homeostatic drive and preservation across species, sleep is a temporary and reversible state of diminished consciousness which is believed to be restorative and necessary for survival."  

— Indira Gurubhagavatula, Associate Professor of Clinical Medicine (Division of Sleep Medicine), Penn Medicine

12. "The natural state of the human mind is asleep, idling so to speak. Wakefulness only occurs when there is sufficient input to alert us from this dream." 

— Nicholas Dodman, Director of Animal Behavior Program, Cummings School of Veterinary Medicine at Tufts UniversityWhatisSleep_Inset1

13. "Sleep is life." 

— Monica Anderson, Department of Psychobiology, Universidade Federal de São Paulo, São Paulo, Brazil 

14. "Sleep is one of the most vital components for a healthy life since it correlates with lower rates of depression, obesity, bullying, car accidents and illnesses, and is associated with more engagement with learning, school achievement, and overall physical and mental health."

— Denise Clark Pope, Senior Lecturer, Stanford School of Education

15. "A reversible state of relatively decreased activity and relatively decreased reactivity to outside stimuli."

— Per Davidson, Psychologist, Lund University, Sweden

16. "In healthy sleep, a person’s body rests, their brain processes while in their mind they have dreams; whereas in disorders of sleep, they breathe fitfully, gasping and choking, awakening throughout the night with anxious thoughts hoping to avoid the terror of nightmares."

— Vincent Mysliwiec, Sleep Medicine Specialist  

17. "Sleep is protected downtime to learn and project."

—Peter Hancock, Psychology Professor, University of Central Florida

18. "Sleep, a dynamic and restorative state of unconsciousness in which our brains are highly active, is vital for mental and physical health, happiness, and life."

— Jennifer Schwartz, Associate Research Scientist, Yale

19. "Sleep promotes the consolidation of newly acquired memories into a more stable, long-term form — and in my opinion, dreams reflect this process."

— Anthony Murkar, Psychologist, University of Ottawa, Canada

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20. "Sleep is when your brain attends to its internal environment, and goes off-line from the outside world."

— Guy Leschziner, Neurologist, Department of Neurology, St. Thomas’ Hospital, London

"Sleep is protected downtime to learn and project."

21. "Sleep is the default state of the brain, an unperturbed state determined solely and characteristically by the internal dynamics of its networks."

— György Buzsáki, Biggs Professor of Neural Sciences, NYU Neuroscience Institute, Langone Medical Center

22. "Sleep is a behavior indispensable for optimal functioning during wakefulness."

— Paul Franken, Center for Integrative Genomics, University of Lausanne, Switzerland.

23. "During sleep, all the updates are installed and your brain reboots: You can put it off for a bit, but your software won’t function properly, and if you put it off too long, the system may crash."

— Carl W. Bazil, Professor of Neurology Director, Division of Epilepsy and Sleep, Columbia University