Just sleep on it. The adage has been woven into the very fibers of our culture as a means to reconcile daily challenges and cope with life’s problems. New research suggests the popular advice has neurological salience, as the brain’s mechanism for waste removal functions most effectively while we’re deep in dreamland.

Mental illnesses, such as Alzheimer’s disease and dementia, are thought to occur because of a buildup of beta-amyloid plaque and tau-based neurofibrillary tangles, two biomarkers whose direct mechanisms for causing diseases are unknown, yet which are routinely suspected in leading to such diseases. Scientists from the University of Rochester Medical Center now argue in their latest study, published in Science, that sleep serves as a means to export cerebral waste before either biomarker has the chance to accumulate.

"This study shows that the brain has different functional states when asleep and when awake," said Dr. Maiken Nedergaard, co-director of the URMC Center for Translational Neuromedicine and lead author of the report. "In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness."

The waking person is endlessly bombarded with huge amounts of sensory information — scents, data, tastes, language — all of which must be dutifully received by the brain. While the commanding organ is designed to field such data streams efficiently, the act is still effortful. Scientists suspect this may be where sleep comes in.

As a biological imperative, sleep pervades nearly all species that inhabit the earth. Even giraffes, which sleep a bleary-eyed 30 minutes a day — and do so standing up with one eye open, scanning for predators — still must get some shut-eye. Sleeping restores bodily function. It energizes the body. But it’s still widely speculated as an evolutionary mistake. Humans are at their most vulnerable when they sleep, so it makes little sense that prehistoric humans, without the modern comforts of home alarms and under-the-pillow revolvers, would voluntarily become unconscious while the sun is on the other side of the earth.

So why does it happen?

"The brain only has limited energy at its disposal and it appears that it must choose between two different functional states – awake and aware or asleep and cleaning up," said Nedergaard. "You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can't really do both at the same time."

Based on the current study, researchers argue sleep helps the brain “take out the trash.” In other parts of the body, the lymphatic system, which is a part of the circulatory system, helps cycle and flush various fluid and fatty acids from tissues and areas of the digestive system. In the closed ecosystem of the brain, a different system performs these functions. Scientists called it the glymphatic system, and it effectively shrinks a person’s brain cells while he or she sleeps so that cerebral spinal fluid can more easily pass through the brain’s open spaces. This fluid then piggybacks on surrounding blood vessels and circulates throughout the rest of the body, ultimately finding its way to the liver, where it can be processed as waste.

Over time, the brain’s failure to remove these waste products causes them to build-up in the neighboring tissues. Plaques and subsequent tangles can accumulate and disrupt healthy pathways, degrading the neural connections within the brain and collapsing the neuron’s transport system. Researchers in the latest study found sleep among lab mice revved up their glymphatic systems 10-fold and significantly reduced the amount of beta-amyloid in their brains.

Moreover, the hormone noradrenaline is less active during sleep. Noradrenaline is responsible for making a person alert and ready, and its cyclical presence and absence during the day, the researchers suspect, make it something of a “master regulator,” capable of controlling how much brain cells contract and expand.

Moving forward from their study, the experimenters hope more information can emerge when it comes to treating neurodegenerative diseases. The reduction of beta-amyloid in the brain may indeed designate sleep as the primary arbiter in preventing the painful slide into mental illness.

"These findings have significant implications for treating 'dirty brain' disease like Alzheimer's," Nedergaard said. "Understanding precisely how and when the brain activates the glymphatic system and clears waste is a critical first step in efforts to potentially modulate this system and make it work more efficiently."

Source: Nedergaard M. Garbage Truck of the Brain. Science. 2013.