Deep Sleep, scientifically known as Slow Wave Sleep (or SWS), is the third sleep stage (N3). In 2008 the sleep profession in the US eliminated the use of stage 4. Stages 3 and 4 are now considered stage 3 or N3.
Deep sleep reduces your sleep drive, and provides the most restorative sleep of all the sleep stages. This is why if you take a short nap during the day, you’re still able to fall asleep at night. But if you take a nap long enough to fall into deep sleep, you have more difficulty falling asleep at night because you reduced your need for sleep. During deep sleep, human growth hormone is released and restores your body and muscles from the stresses of the day. Your immune system restores itself.
The EEG (electroencephalogram), which measures brain electrical activity, is made of intense slow waves (their frequency fluctuates between 0.75 and 4Hz). We can therefore estimate the depth or the quality of one's 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, helps decreasing stress effect and increasing essential hormones secretion, like growth hormone. Studies showed people deprived of Slow Wave Sleep are more lethargic, experience muscle aches and are more prone to be insulin resistance (pre-diabetic state).
It is significantly harder to wake up during Slow Wave Sleep than in any other sleep stage.
Evolution of deep sleep with age
The quantity and the intensity of Slow Wave Sleep (SWS) vary 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 decreases and can even disappear for some individuals as we get older.
Here’s the EEG of a N3 detected for a young man (20-50 years old)
Here’s the EEG of a N3 detected for an elderly man (78 years old)