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 are 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 Related Topics). 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.