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 sensory details we need to remember and what we can let go of.

Dr. Laura Lewis and her team of researchers at Boston University found out that during deep sleep, toxic compounds in the brain like beta amyloid, found in the brains of Alzheimer’s patients, were wiped out.

Lewis wondered why this process only t...