Although at first glance it might seem obvious what sleep is, it remains tantalizingly difficult to pin down in definitive terms. Is it, as the Roman poet Ovid had it, “chill death’s likeness” or is it, as Shakespeare so eloquently described it, “nature’s soft nurse” (for the record, Shakespeare, as so often, was closer to the truth).
We know that we normally lie down and close our eyes when we go to sleep, and that this normally occurs at night, but are these actually fundamental to the process of sleep, or merely ancillary aspects? We might be tempted to specify that sleep is a passive state of unconsciousness, a suspension of our normal bodily activities, as was assumed for millennia. However, we now know that this is far from the truth, and that sleep is actually a complex and far-from-passive process of active internal restoration, recuperation and reconsolidating which is essential for our health and well-being (see the section on Why Do We Sleep for more details).
Moreover, it is hardly possible to talk of such a simplistic thing as “sleep“. Our nightly sleep is made up of several sleep cycles, each of which is composed of several different sleep stages, and the physiological and neurological differences between the two main types of sleep, non-REM and REM are almost as profound as the differences between sleep and wakefulness (or, for that matter, night and day). If we then consider the very different modes of sleep experienced by different animals (see the section on Sleep in the Animal Kingdom), the question becomes even more confused.
What, then, are the necessary prerequisites that we need to take into account for a categorical definition of sleep? In the sections that follow, we will look at this, as well as looking at what sleep is not, and also where the word itself actually comes from (as well as other sleep-related words).
The Oxford English Dictionary defines sleep as:
“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:
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.”
It is most important to note that sleep is not just a passive process – a time when our normal mental and physical activities shut down or are put on hold for a while – as had been assumed for thousands of years before the modern scientific era. Rather, (as we will see, particularly in the sections on How Sleep Works and Why Do We Sleep), sleep is actually an active and dynamic physiological process, essential for our normal motor and cognitive function.
Sleep can usually be recognized by the state of quietude and immobility that accompanies it, and even by the recumbent (lying down) posture usually adopted for it. But neither of these properties is a sine qua non for sleep, and they are essentially incidental results of the reduced muscle tone and relaxation of the skeletal muscles that accompanies sleep. Also, it is quite possible to achieve such a state of restfulness and quietude without being asleep, either by meditation or just by sitting quietly.
Similarly, sleep normally occurs at night, with the eyes closed, etc, and these may indeed be the outward signs most associated with sleep. But once again, these properties do not define sleep (and are not part of our Definition in the previous section). Sleep clearly can and does occur at other times, particularly in the case of shift work and naps (both of which are explored in more detail in the section on Other Sleep Resources).
Likewise, some animals are able to sleep with their eyes open, or in some cases with one eye open and one eye closed (unihemispheric sleep). Indeed, although sleep may be observed in all mammals and birds, as well as in most (but not all) reptiles, amphibians, fish and even insects, in some cases it may be quite difficult to reconcile an animal’s physiological and behavioural patterns with what we traditionally think of as sleep (see the section on Sleep in the Animal Kingdom).
As mentioned in the previous section, sleep is different in nature from superficially similar states such as hibernation, estivation, coma and general anesthesia. There are also similarities between sleep and the state of hypnotic trance, which is often claimed to be analogous in some ways to REM sleep.
But there are also some important distinguishing factors that differentiate a hypnotic trance from sleep: a person under hypnosis is quite capable of conversing and reasoning with another person, and of responding to suggestions; hypnosis is usually deliberately induced by another person, rather than occurring naturally; brain wave activity during hypnosis is almost identical to that of the waking state; muscle tone and tendon reflexes are normal during a hypnotic trance, and not suppressed as they are in sleep; etc.
However, the main thing that sleep is not is wakefulness, which in fact is usually defined in the negative, i.e. as the absence of sleep.