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 reconsolidation 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 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).