Better depressed and safe, than happy and sorry.
"Researchers argue humans did not evolve to be consistently happy but primarily survive and reproduce. Evolution, they say, placed an advantage on depression, preventing humans from engaging in risky or hopeless situations."
"The fluid contains chemicals that protect nerve cells from damage, and unlike blood, has no immune cells, which may contribute to brain damage. “If this works for brain cells it will also work with some less sensitive organs with some tinkering,”"
Mental activity during the day produces buildup of beta-amyloid plaque, the protein known to play a role in Alzheimers disease. Sleeping cleanses the brain of this buildup, but poor sleep quality means less of this cleansing effect.