In deep sleep, the cerebrospinal fluid acted like waves on a lake splashing over the brain. During this period, the neurons would gradually stop firing, requiring less oxygen which in turn meant less blood flow to the brain. As the blood flow decreased, the rest of the space would fill with cerebrospinal fluid.

When awake, brain blood levels don’t drop enough to allow waves of cerebrospinal fluid to circulate around the brain and clear out all the metabolic byproducts that accumulate as we function, like beta amyloid, a protein that disrupts neural connectivity in patients with Alzheimer’s.

The research shows that sleep may have other, more sophisticated purposes rather than simply recharging the body after a day of hard work or strenuous decision making. Recent studies suggest that sleep is paramount to our ability to forget things — that toxins in the brain help us filter what sen...