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Sleep/rest is something every animal does and needs; without it, and specifically REM sleep, humans do not survive.
This really hits home on just how important sleep is and even the type of sleep is for a human to function properly. So for an athlete, who is training regularly, you can appreciate how important it is, not only for physical repair and recovery but also mental health and stability.
A lot goes on during our sleep phase in terms of both physical repair and psychological processing. Studies show that the brain is very active during certain phases of our sleep and can appear/correlate almost identically to an awake brain.
So lets dive in a little bit on sleep phases or stages which are often divided into 4 categories:
Stage 1 - Early stages of sleep. One can still perceive surroundings and you can easily be awoken. This stage is identified specifically by changes in brain waves to indicate 'sleep' rather than just resting
Stage 2 - Deeper sleep now in terms of awareness of surroundings. The brain does still recognize surroundings (sounds, light) but you do not wake. Recent studies show that we spend roughly 50% of our sleep in this stage
Stage 3 - Deepest sleep where you are pretty much unconscious and do not wake from even loud sounds around you. This is where slow waves are identified in the brain and where most of the physical repair takes. The immune system is particularly active during this phase. There is a lot more sleep in this stage during the early parts of the sleep cycles (more on cycles below), and this time is reduced in later stages of sleep.
Stage 4 or REM - This is considered the most important stage of sleep. It is considered a period where a lot of cognitive and emotional reconciliation is happening. The brain is highly active and very similar to an awake brain but you are asleep and in paralysis. There are short burst of this stage during the earlier parts of sleep but they get longer during the later parts of sleep.
So this then raises the concept of sleep cycles. We do not simply migrate sequentially from one stage to next and then back up once during a night's sleep. Instead we cycle through pretty much all stages several times and can even skip stages.
During the night, the cycles change as our predominant stage changes. For example, we spend predominantly more time in deeper (stage 3) sleep with short bursts spent in REM stage during the early parts of the night. That generally shifts to more time being spent in REM stage sleep and reduced time in stage 3 as we progress through the night.
It is therefore evident how the different types of sleep or lack thereof, can affect us differently. Reduction in sleep or sleep quality generally affects mostly emotional and cognitive changes as many might experience during periods of jet-lag. You feel physically ok but it more evident with the head feeling foggy.
There is much more to this topic. For a full, in-depth look at sleep, how some people are programmed, what sleep gadgets work and more, head on over to The Real Science Of Sport podcast.