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How Sleep Cleans the Streets of Your Brain

Have you ever seen the streets of a major city being cleaned late in the wee hours of the night? In every major city, in the dark quiet of the empty streets, one can usually hear garbage trucks and street sweepers slowly cleaning up the mess from another day of controlled chaos.

The same thing is going on in your brain, especially during the first 5 to 6 hours. During the day waste products produced by the 100 billion cells of our brains builds up in and around all those cells. Street cleaning of cities has to wait until nighttime when traffic is low so there is room to clear out the garbage. The same thing happens in the brain. A particular cell, called an astrocyte (because it looks like a star) starts to shrink at night, creating spaces between all the cells that allows the fluid filled with the day’s metabolic garbage to collect for pick up. But what is the brain’s sanitation truck?

Most of us are familiar with Rapid Eye Movement (REM) sleep as being the part of sleep where we dream. REM sleep is total neurologic chaos and is mostly in the last 2–3 hours of a typical 8 hour sleep cycle, which is why we wake up and can often remember our crazy surreal dreams.

The first 5–6 hours of sleep is nonREM sleep or NREM. This is when the those astrocytes shrink and open up the streets for the fluid filled with day’s metabolic garbage to get ready to be cleared out. Unlike REM sleep, the neurologic pace is slow and languid with slow steady electrical waves that rhythmically pass, over and over, from the front of the brain to the back. Until now, it wasn’t clear exactly how those waves participated in clearing the metabolic garbage from the streets of our brains.

Nina Fultz and her colleagues from Boston University and Harvard Medical School performed detailed MRI scans on 13 people while they were in NREM sleep. It is important to keep in mind that the brain, unlike other organs in the body, is stuck in a tight box — the skull — so it has a miniscule ability to enlarge. Thus, the ability to “hose out” the waste by just washing down the littered neurolgic streets with a large volume of fluid is not an option.

Instead, the brain has devised a workaround for the tight space problem. Once we enter NREM sleep, the slow electrical waves sweep the brain triggering an outflow of just enough blood which makes just enough room for a pulsatile inflow of cerebrospinal fluid into the ventricles. So the sequence is: the astrocytes shrink, the fluid filled with metabolic garbage collects in the streets of your brain, and the electrical waves start the overnight process of pumping out the dirty cerebrospinal fluid. It is a slow process that takes 5-6 hours.

The video shows the changes in blood flow (red) and cerebrospinal fluid flow (blue) and how they alternate in a rhythmic manner driven by the electrical waves of NREM sleep.

Bottom line: the first 5–6 hours of sleep are critical for cleaning up your littered neurological streets. Some of the neurologic litter is not harmful and just routine housekeeping, sort of like dusting. But some of the litter is toxic to our brain cells, especially over time.

Two toxic proteins in particular are cleared during NREM sleep: beta-amyloid and tau. Both of them are know to damage nerve cells and they are strongly associated with Alzheimer’s disease where abnormal accumulations of both proteins are seen.

Just pulling one “all-nighter” by staying up 24 hours leads to a significant accumulation of beta-amyloid in the hippocampus, a key brain structure critical for memory formation.

As a surgeon who out of necessity honed and embraced the skill of learning how to cope with sleep deprivation, I now see how misguided this paradigm was. Not only is sleep critical to help fend off the ravages of aging, but it is also critical to our ability to be at our best, day after day. Sleep is the best mind clearing and productivity enhancing drug on the market.

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