Your brain’s nightly cleanse

Because of improved imaging techniques, scientists are learning new things about how our brains work. They’ve known that brains generate waste but didn’t know, until recently, how the waste was removed. (In the rest of our bodies, garbage removal is handled by the lymphatic system.) As our brains work throughout the day, they generate waste—excess proteins and other molecules that can be toxic if not removed. Among those proteins are amyloid beta and tau, key drivers of Alzheimer’s disease.

It’s only been since 2012 that scientists have discovered a circulatory system in the brain, one in which cerebrospinal fluid flows from the spinal cord into the brain. In the brain, the cerebrospinal fluid mixes with other fluids in the spaces around the brain’s cells, collects the waste, then leaves the brain, taking the garbage with it.

In 2013, scientists learned that this brainwashing was most active and efficient during sleep, suggesting that the garbage collection process is one of the critical functions of sleep. The electrical waves that sweep through the brain during sleep propel cerebrospinal fluid in and out of the brain. The brain is not less active during sleep; it’s differently active.

The discovery that waste clearance is an essential function of human sleep suggests that a dysfunction in this cleansing system is a likely cause of many neurological and psychiatric disorders. It’s long been known that accumulation of amyloid and tau proteins leads to Alzheimer’s, but the link between sleep and a faulty waste-clearance process was not obvious. One clue is the fact that conditions such as traumatic brain injury, which is associated with Alzheimer’s, interfere with waste clearance.

None of this is to say you should worry about your sleep! Just be grateful that it’s being put to good use.