Physiological Aspects of Sleep

In response to the many messages from you all, requesting that we help organize your knowledge about the medical aspects of sleep, we’ve decided to launch additional notes concerning not only interesting facts about sleep but also some textbook knowledge in an accessible and user-friendly form.

Physiological Aspects of Sleep – What Is Going On With Our Body When We Sleep 

In order to examine the quality of sleep doctors use a machine called polysomnography. It records brain waves, eye movements, and muscle tension. It is crucial to record all these parameters to distinguish sleep from awakening and to differentiate between the sleep phases. Based on these three variables, we divide sleep into two major phases – REM (rapid eye movement) and NREM (non-rapid eye movement). Together, the REM and NREM phases are called the cycle of sleep. This lasts on average 90-110 minutes.

To be more precise, we can divide the NREM phase into 4 subphases. In the 1st subphase, we can see slow eye movements. The 1st and 2nd subphases are sometimes called shallow sleep phases, whereas the 3rd and 4th subphases are called stable or deep sleep phases. Response threshold is the lowest in subphase 1 and the highest in subphase 4, which means that it is easy to wake us up in the shallow (1st and 2nd  NREM subphases) and quite difficult in deep sleep (3rd and 4th NREM subphases). As we progress through the NREM subphases the antigravitational muscle tension (which enables us to hold a vertical position) fades.  

The REM phase, in which we can observe rapid eye movements, is quite different. It occupies only 20-25 % of our night sleep time, but up to 85% of dreams occur in this phase. What is interesting is that the great majority of these dreams are completely chaotic, eerie and relating to situations that are highly unlikely to happen. For example, escaping from a chasing pack of tigers or jumping from a waterfall, whereas during the NREM phase dreams are more likely to contemplate situations which actually happened, or probably will happen in the future – for example, our plans or our past. Some scientists presume that REM dreams are present as the inheritance from our ancestors, in order to let our brain analyze and teach itself how to behave in situations which may put our life in jeopardy. This is probably why in the REM phase all our skeletal muscles are atonic – we literally cannot move a muscle.

Other biological functions also differ between REM and NREM phases. In the NREM phase, our heart beats more slowly (especially in the 4th subphase), blood pressure and ventilation are lower and, generally, less blood flows through the brain due to the fall in the metabolic rate of this organ (especially in deep sleep). In the REM phase, all these variables are fluctuating and cannot easily be qualified as higher or lower.

What differentiates us from other mammals is the fact that our metabolic rate during the REM phase is not lower than during the NREM phase. This can be explained by the greater participation of the brain in this phase, however, in general, the metabolic rate of our body is 5-15% lower during sleep than when we are awake.

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