The function of Sleep is Rest.

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After reading this article it now becomes crystal clear that the true function (prevention of death) of sleep is related to the immune system.  How the researchers haven't figured this out yet is beyond me.

Simple animals like flies develop Reactive Oxygen Species filling their guts. Other animals like mice develop leaky gut and bacteria invade the body from the gut, basically causing sepsis.  Another thing is adrenal glands become enlarged in sleep deprivation which is because adrenals emit steroids which reduce inflammation.  Also antioxidants are known to keep flies alive during sleep deprivation, antioxidants also reduce inflammation.

What do these have in common?  What happens is that the bodies' immune system keeps Gut Microbes in check.  With a lack of sleep this immunity cannot be sustained.  Also what happens is the immune system is retrained during sleep.  Without sleep the immune system goes into dysfunction where it is not reprogrammed with more information, it fights in a dysfunctional manner causing basically autoimmune inflammation.

So sleep allows immune cells to be reprogrammed with new more accurate and up-to-date information.  Synthesizing information.  Basically during sleep the immune system takes a break.  This is also why people are most likely to die during sleep, since the immune system is resting and bacteria do overgrow especially during REM sleep.  But once the immune cells get rest they are more likely to react to intrusions in a more "level-headed" and effective way.

Once researchers figure out that Sleep works by letting the immune system rest, their heads will explode because we are right back to the same question..."why do immune cells need rest to function properly?"  The truth is that sleep is a time for "software updates" and synthesis of observation and this is played in a large part by DMT.  Also the inability to ever find a "root cause" of what sleep does, will be because scientists will not allow themselves to discover that these software updates come from a non-physical place - either God or the Collective Unconscious or the Earth or Nature or the Universe - and pragmatic scientists will never allow their research to go there, they must always find a physical cause.  There are physical causes but especially in the case of sleep and REM sleep, something non-physical guides the nightly updates.  When the creature awakens (and their immune cells) they interpret the information they received during sleep into a more efficient use of their time and resources.  When immune cells don't get reprogrammed they fail in battling viruses and bacteria - when the human brain itself doesn't get reprogrammed, they fail in effectively dealing with life's challenges.  The latter may not necessarily cause direct physical death, but it can cause reduction in quality of life and lowered life expectancy from accidents (and all causes when you consider the immune system dysfunction which also happens in parallel).

Personally I think that REM sleep is a sort of "natural social media" where creatures and entities can communicate with eachother and teach and learn things over vast distances.  What is the physical media that allows this?  It is called the Aether which is a fabric of relatively stationary photons that fill the universe and cannot be seen by typical means.  A sort of "Dark Matter" but one that creatures, especially from the brain and other structures, can interact with and send messages and communication.