How Deep Sleep Helps Your Brain “Clean” Itself

deep sleep

Whether we look forward to it or not, most of us think we understand the value of a long, uninterrupted sleep every night — often after we’ve gone without it for too long. Dr. Laura Lewis and her team of researchers at Boston University have sacrificed a great deal of sleep too — but they understand the importance of sleep a bit better than most, thanks to the long hours of lab tests they’ve done, sometimes running until three in the morning.

The long nights of work gave her a feeling she described as jet lag without crossing time zones, but ultimately paid off — as she published her results in the journal Science. The research shows that sleep may have other, more sophisticated purposes rather than simply recharging the body after a day of hard work or strenuous decision making. Recent studies suggest that sleep is paramount to our ability to forget things — that toxins in the brain help us filter what sensory details we need to remember and what we can let go of.

Lewis’ research has been charting what happens to us as we drift off to sleep — with our brains drifting through phases of sleep every hour. Typically we start out in a light slumber, drifting off — before entering a deep and dreamless sleep. From there, we typically enter REM (rapid eye movement sleep, where we often experience the most vivid of our dreams. As intriguing as dreams may seem to us — as we try to retrace each detail and strain to decipher their meaning, Lewis noticed something else taking place during REM in lab mice. While her test subjects slept, toxic compounds in the brain like beta amyloid, found in the brains of Alzheimer’s patients, were wiped out.

Lewis wondered why this process only took place during sleep and more importantly, what mechanics were involved. She suspected the source to be cerebrospinal fluid, a clear, watery substance that flows around the brain using valves. She recruited human subjects to sleep inside an MRI machine, and had them stay up late the night before so they would be ready to fall asleep as her researchers conducted their tests. A cap fitted with electrodes was put on and measured the state of sleep they were in, while the MRI read the blood oxygen levels in the brain, as well as the levels of cerebrospinal fluid.

“We had a sense each of these metrics was important, but how they change during sleep and how they relate to each other during sleep was uncharted territory for us,” said Lewis.

deep sleep

During periods that the brain was not in REM, she found that the cerebrospinal fluid acted like waves on a lake, gently splashing over the brain. During this period, the neurons would gradually stop firing, requiring less oxygen which in turn meant less blood flow to the brain. As the blood flow decreased, the rest of the space would fill with cerebrospinal fluid.

Because neurons are regularly active throughout the day, while we work, blood levels in the brain don’t drop to a significant enough level to allow these waves of fluid through. Therefore, byproducts like beta amyloid are allowed to build up while we’re awake.

Neurons don’t all turn off at the same time when we’re awake. So brain blood levels don’t drop enough to allow substantial waves of cerebrospinal fluid to circulate around the brain and clear out all the metabolic byproducts that accumulate as we function, like beta amyloid, or tau, a protein that disrupts neural connectivity in patients with Alzheimer’s.

These types of plaque have been targeted with drug therapies in the past, but researchers like Lewis are hoping that further research will be able to define the pathways of cerebrospinal fluid more clearly — perhaps finding mechanisms that can increase the amount of fluid released into the brain at night. It’s safe to say that this research merely scratched the surface, as Lewis is now wondering what impacts the brain in the other stages of sleep and what further clues might they hold.

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