As we fall asleep, the brain begins clearing out waste.
It operates like a late-night laundry service, with all the water valves opened and washing machines running at full capacity to remove dirt from piles of clothes, flushing the wastewater into the drain.
https://www.zerohedge.com/medical/your-brain-flushes-out-waste-every-night-heres-how-help-it-clean