Sleep is divided into two broad types: rapid eye movement (REM) and non rapid eye movement (NREM) sleep. Each type has a distinct set of associated physiological and neurological features. NREM is further divided into three phases: N1, N2, and N3,
- NREM phase 1: This is a stage between sleep and wakefulness. The muscles are active, and the eyes roll slowly, opening and closing moderately. This stage is sometimes referred to as drowsy sleep and is easily awaken. Sudden twitches also known as myoclonus may be associated with the onset of sleep during N1.
- NREM phase 2: In this stage, it gradually becomes harder to awaken the sleeper; During this stage, muscular activity decreases, and conscious awareness of the external environment disappears. This stage occupies 45–55% of total sleep in adults.
- NREM phase 3: Formerly divided into stages 3 and 4, this stage is called slow wave sleep (SWS). The sleeper is less responsive to the environment; many environmental stimuli no longer produce any reactions. The absence or shortening of SWS impairs how refreshed you feel in the morning.
- REM: The sleeper now enters rapid eye movement (REM) where most muscles are paralyzed. An adult reaches REM approximately every 90 minutes, with the latter half of sleep being more dominated by this stage. Sleep proceeds in cycles of REM and NREM, usually four or five of them per night, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.
There is of course variability in this pattern. If you have disturbed or under sleep, our bodies attempt to compensate by putting us into REM sleep earlier, primarily by shortening our SWS. The problem is this shortened cycle translates in less growth hormone production, which leads to problems.
To ensure you are achieving quality sleep experts recommend sleeping on a custom built/prescribed mattress tailored to your individual body type.
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