While we’re asleep at night, our brain is busy doing maintenance. The glymphatic system–the brain’s waste disposal mechanism–flushes away excess cerebrospinal fluid containing harmful proteins that otherwise build up slowly in the brain.

The brain's waste-removal process is "like turning on the dishwasher," a neurologist says, but common sleep medications may harm it

Blood vessels in the brain rhythmically constrict and dilate to drive waves of cleansing fluid through the organ.