Deep Sleep, (scientifically known as Slow Wave Sleep or SWS) is the the third sleep stage N3. The EEG (electroencephalogram), which measures brain electrical activity, is made of intense slow waves (their frequencies are comprised between 0.75 and 4Hz). We can therefore estimate the depth or the quality of sleep by measuring slow wave density, the presence of stage N1, the number of awakenings…
Slow Wave sleep is associated to a diminished physiological activity but is actively regulated by our brain. Broadly speaking, it has a homeostatic function : it plays a role in body and cognitive processes restoration. It cleans out the brain, diminishes stress effect and secrete essential hormones (like growth hormone). Studies showed people deprived of Slow Wave Sleep are more lethargic, experience muscle aches and are more to insulin resistance (pre-diabetic state).
It is significantly harder to wake up during Slow Wave Sleep than in any other sleep stage.
Too few deep sleep?
The quantity and the intensity of deep Slow Wave Sleep (SWS) varies from one sleep cycle to another and even from one night to another.
We often talk about SWS pressure, because the more you are sleep deprived, the more easily, rapidly and intensely you will fall in deep sleep. In such cases, RMES quantity decreases to extend SWS.
Moreover, SWS varies a lot during your life. Although newborns spend tremendous amount of time in deep sleep, it is less present as we are getting older. It can even disappear totally for some retirees.