Sleep is heterogeneous, in such a way that we can distinguish between slow wave sleep (non REM) and paradoxal, or Rapid Eye Movement (REM) sleep.

Furthermore, on the circadian waking-sleeping rhythm is superimposed a shorter periodicity rhythm – one talks of an ‘ultradien’ rhythm. In the order of 90-100 minutes, it characterises a sleep cycle, in other words a succession of slow wave and paradoxal sleep. A normal night is made up of four or five of these cycles, rarely six. Within each phase the respective amounts of slow wave and paradoxal sleep can fluctuate. Sixty minutes against forty, thirty against seventy, etc.Slow wave sleep can be further divided, and we have been able to distinguish four stages within it thanks to so-called polygraph studies.

These are based on different parameters, which represent markers of the different stages of slow wave sleep: eye activity, examined closely by oculography; that of the muscles, studied through myography, or that of the brain, revealed by electro-encephalography.The first stage of slow wave sweep is in effect a state of somnolence. The eyes of the subject are half-closed, the eyeballs travel slowly from left to right; the subject can still speak, but becomes confused. Stage 1 is essential, in that it allows the autonomic nervous system to check that all the individual’s vital functions are secure.

As certain electro-encephalographic characteristics show, stage 2 is the prop for light sleep, to which corresponds a cerebral metabolism nearly as intense as that when the subject is in a state of active wakefulness. It thus offers little in terms of recuperation. Nonetheless, it consolidates a state of unconsciousness which, according to an expression by Professor Robert Poirrier, in charge of the Sleep Laboratory at the Liège University Hospital Centre, allows the subject to fall into the large tank of deep slow wave sleep (stages 3 and 4), which will be divided up in order to fit in with the cycles mentioned earlier and to coexist with episodes of paradoxal sleep. It should be noted however that this new classification of sleep no longer distinguishes between stages 3 and 4.