Sleep cleans up the brain
A similar system to our body's lymphatic system, exists in our brain - and its called the glymphatic system. A day's worth of activity for the most metabolically active organ certainly generates numerous byproducts, that at some point, have to get flushed out. It turns out that during sleep this system is in overdrive (a 60% increase in the drainage system). I found this to be a groundbreaking study, which essentially reveals why the brain may have to be "off line" to do a massive clean up of this scale. I am eagerly awaiting further research into the particular phase of sleep involved, but I suspect slow wave sleep is the answer - and there may be pharmacologic ways of enhancing this.
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