Nothing "happens" by itself at all when you sleep.
Important to understand: Your body (and mind) are not just some kind of passive biological automatic machine, to which
things happen.
You are a living being. There is nothing automatic in keeping yourself alive. Contrary, living is a continuous combined
non-stop job, awake and asleep.
Remember, sleep is an activity, something you do.
Activities simply don't ever happen. You do them efficiently to
accomplish their purpose or you don't. That is why medicine and psychiatry principally can do nothing successfully to improve your sleep. You
have to improve your sleep activity yourself. The good news: You certainly can do that.
You may ask:
"How could Sleep possibly be an activity, if I explicitly do absolutely nothing, eyes closed, lie immobile and unconscious in
bed when sleeping?"
When you sleep you keep doing many things. You keep pumping blood around with your heart. You keep breathing, billowing
air in and out of your lungs. All this does not happen to you. You actively perform it, competently directed by your brain.
The only thing you obviously don't do while sleeping is looking and moving around.
You immobilize most of your 625 muscles, take load off of your bones and joints, so that you can service and restore them.
You service your eyes and de-clutter your memory. That's why your conscious awareness must be offline to sleep.
You must take yourself "off the road" and "out of the daily war zone" to do a decent inner sleep job on yourself.
Nothing of this happens on it's own. Sleeping is your non-conscious vital daily high-intensity self-maintenance behaviour.
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Next Question in Line: What do I do when I Sleep ?