The purpose of sleep has been the subject of debate for millennia. The necessity of sleep across animal species suggests that sleep plays a vital function.  One of the greatest mysteries in biology is why sleep matters. Why is sleep restorative?  And, why does the lack of sleep impair brain function?  According to Dr. Lulu Xie, of the University of Rochester Medical Center, sleep deprivation reduces learning, impairs performance in cognitive tests, prolongs reaction time, and in the most extreme cases, can lead to dementia and even death.

Without sleep, scientists have found that lab mice die after two weeks. Although it is still not precisely known why sleep is necessary, Dr. Xie, and her team, are uncovering the mystery of how sleep enhances the removal of waste products from our brain.

Amazingly, the brain has a simple, yet elegant, way to cleanse itself, while we sleep

During our waking hours, the brain produces toxic byproducts as a result of regular, metabolic activity.  In the short-term, these metabolic waste products can interfere with normal cell communication, and in the long-term, can cause irreversible damage to brain cells.  One such neurotoxic protein is beta-amyloid, a sticky compound that can accumulate in the brain, disrupting communication between brain cells, eventually killing them. Accumulation of beta-amyloid protein in the brain is one of the culprits thought to be one of the causes of Alzheimer’s disease.

The brain’s mechanism for clearing out toxic byproducts involves the Cerebrospinal fluid (CSF) that fills the interstitial space surrounding neurons in the brain.  CFS continually travels throughout the brain collecting waste byproducts and transports them to the lymphatic system for ultimate disposal and degradation by the liver. Dr. Xie’s research showed that this cleansing process dramatically increases in the brains of mice while they sleep, in a simple, yet elegant way.

Dr. Xie’s team reported that while the mice were sleeping, the interstitial space of their brains, increased by 60%, allowing for a dramatic increase in the volume of CSF and the clearing of toxic byproducts. The team was able to observe this process by injecting radioactive tracers into the CFS of mice so that they could watch as it flowed through the brain. They found that when the mice were awake, there was more resistance to CSF.  However, when they were asleep, the interstitial space between brain cells increased by as much as 60%, a vastly larger space, allowing for significantly more efficient removal of waste products.

Why sleep matters is still one of the greatest mysteries in biology, but Dr. Xie’s team gives compelling evidence to answer the questions of why sleep is restorative, and, as important, why the lack of sleep impairs brain function.


Xie, Lulu, et al. “Sleep Drives Metabolite Clearance from the Adult Brain.” Science, vol. 342, no. 6156, 2013, pp. 373–77, doi:10.1126/science.1241224.