“a condition of body and mind which typically recurs for several hours every night, in which the nervous system is inactive, the eyes closed, the postural muscles relaxed, and consciousness practically suspended”.
Merriam-Webster prefers the more pithy definition:
“the natural periodic suspension of consciousness during which the powers of the body are restored”.
The MacMillan Dictionary for Students offers:
“sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles”.
A slightly more scientific definition can be found in Stedman’s Medical Dictionary:
“a natural periodic state of rest for the mind and body, in which the eyes usually close and consciousness is completely or partially lost, so that there is a decrease in bodily movement and responsiveness to external stimuli”.
The common threads in these descriptions, which appear to be necessary elements in the definition of sleep, are that it:
- is a naturally-occurring state
- is periodic and recurring
- involves both the mind and the body
- involves the temporary suspension of consciousness
- involves the relaxation and inactivity of muscles
Sleep is also a state in which the person or animal is less responsive to outward stimuli than in a usual waking state, but to a lesser extent than in the case of hibernation (in which an animal’s metabolic rate, temperature and breathing are all significantly reduced, and external stimuli may have little or no effect) or coma (in which unresponsiveness is complete and arousal may never occur) or the state induced during general anesthesia (in which a person cannot be awakened and fails to respond to even painful stimuli). An important aspect of sleep, therefore, in contradistinction to hibernation or coma for example, is that it must be easily and immediately reversible, and this should also be part of our definition.
Taking all this into account, our definition of sleep might then look something like:
“a naturally-occurring, reversible, periodic and recurring state in which consciousness and muscular activity is temporarily suspended or diminished, and responsiveness to outside stimuli is reduced”.