Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

Some of these garbagy molecules such as beta-amyloid clog up neurons.13 Others such as adenosine make us sleepy as they accumulate (and are counteracted by caffeine).14 Getting rid of these waste products, however, is a challenge. Whereas tissues like liver and muscle wash out metabolites directly into blood, the brain is tightly sealed off from the circulatory system by a blood-brain barrier that prevents blood from coming into direct contact with brain cells.15 To rid itself of waste, the brain evolved a novel plumbing system that relies on sleep. During NREM sleep, specialized cells throughout the brain expand the spaces between neurons by as much as 60 percent, allowing cerebrospinal fluid that bathes the brain to literally flush away this junk.16 These opened