We all think that we know exactly what sleep is but experts are divided on how to define it. How does this sound to you: "an accumulation of phenomena which satisfy varied interpretations of physiological, motor and sensory criteria". Gobbledygook isn't it? Particularly when during what we all looked upon as normal sleep one or more of those criteria can be absent; people can think in their sleep, some people walk or even talking their sleep, people can train themselves to react instantaneously to external stimuli even when in the deepest sleep.

Do we define sleep as an absence of consciousness together with a complete relaxation of the body? Not really, because a coma, or hibernation, could both be described in this way also. The difference between these and sleep is that the latter is easily reversible, and you can add to that the fact that it is a regular condition and occurs frequently and is normally quite predictable, in that we tend to fall asleep at regular fixed times. The one thing that can throw this system into complete chaos is a change of time zones which means that the body clock is brought out of kilter and doesn't know whether it is sleeping or waking time! This situation can cause considerable upset to the metabolism which can last for many days or even weeks.