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The definition of sleep – folk, ancient – is a behavioral one. It is not scientifically invalid, and it is straightforward. A person is asleep when he or she appears not responsive to external cues (e.g. doesn’t answer to his or her name). Most sleepers have closed eyes and are in a relaxed bodily position.
Exceptions are when the sleeper does not wake up over long periods of time (is in a coma) or is unconscious because of trauma like getting hit in the head.
Moat sleeping bodies are in a relaxed lying or sitting position (although short sleep periods can occur when a person is standing.) The movement of the body and eyelids and the rate of breathing are not wholly indicative of sleep. They can change with the stage of sleep and the manifestation of disorders. We can say, however, that generally the body is motionless or not moving in a manner that suggests direction or intention and that breathing is slower and deeper than during waking.
Talking may occur during sleep, but it is usually not coherent. Sleep is also more common during nighttime and after a person has been awake for a long time.
Is this behavioral definition adequate? Yes, for the most part. Even in scientific study and medical diagnosis situations, this is good enough to say some is sleeping.
An actor could fake sleep and fool observers. This might be easy to do over a short period of time, but only someone who has really observed sleepers can fake the various stages – and an actor would not be able to fake the eye fluttering of REM sleep, but stages 2 and 3 might be fakeable.
Parasomonias are disorders that transcend the line between sleep and waking. When a person is asleep he or she manifests a parasomnia it might seem at first sight that they are waking, but an external observer can generally tell pretty soon that the person is actually asleep. This is one reason parasomnias seem weird to us – a person seems both asleep and awake and engages in sleep walking, sleep talking, limb movements, etc.
The electrophysiological definition of sleep is based on EEG and polysomnogram tests. EEG indications of scalp volatage differences can be classified as waves and scientists know the characteristic waves of the various stages of sleep. Stages 1, 2, and 3 are different enough that even an untrained observer can distinguish between them and the EEG of a waking brain. REM (stage R) EEG readings are much more like waking and to an untrained eye they could be confused for those of an awake person. An experienced sleep technologist can usually tell the differences. In any case, other readings that show up on the polysomnogram can be used to identify REM sleep (e.g. the skeletal muscles do not move).
Some criticize that (electrophysiological) definition of sleep is too human-centric. While humans naturally look to their own species first, animals sleep, too, and examination of animal sleep can yield insight into human sleep.
These definitions are fine and workable. They work almost all the time. One glitch is when you considered localized sleep in parts of the brain or microsleeps. Perhaps in the future a more comprehensive understanding of sleep will allow a fuller definition.