In the course of their normal duties, every cell in our body creates waste byproducts. Just left-over bits of molecules that have been partly used and then discarded. Like microscopic bread crusts. One of the purposes of the white blood system (also known as lymph) is to remove these waste byproducts — just like stormwater drains in a city.
Revealed: The brain’s sewage system
Did you know that this white blood aka lymph has it’s own vessels, similar to veins and arteries, that carry it around the body? Among other jobs, lymph percolates through our tissues and then collects in tiny vessels (like gutters) where it is channeled into larger ducts and eventually joins up with the red bloodstream.
Every part of the body has lymph vessels, except the brain. Curiously enough, until recently no one knew how, or even whether, the brain could get rid of the waste that it creates every day. This is important because the build up of waste products in the brain is known to be the primary cause of degenerative diseases like Alzheimer’s and possibly Parkinson’s.
Now we’re coming to it.
In 2012, scientists discovered that our brains do have a waste-removal system, but it’s entirely different than the rest of our body. Instead of using separate lymph vessels, the brain trickles its waste metabolites along the outside of tiny blood vessels in the brain.
That’s just weird if you ask me. But even stranger, some other scientists have now discovered that what drains the brain of waste is not just a passive trickling effect. It turns out that tiny “smooth” muscles in the brain’s blood vessels help to squeeze the lymph along. Also, the pulse of tension and release while we breathe acts like a pump to shift the brain’s white blood, or “glymph” as it’s now being called, along the tiny blood vessels of the brain to sites where it can be returned to the (red) bloodstream. 
Now here’s where gentleness comes in: both of these forces (smooth muscle contractions and breathing) involve a complex dance between parts of our involuntary nervous system — the part of our brain, spinal cord and nervous system that we don’t consciously control.
Fight or Flight vs Rest and Digest
You’ve probably heard of the “fight or flight” reaction — a state of high alert that we all can access in the presence of danger. That fight or flight reaction is initiated by our involuntary nervous system. But the involuntary nervous system also has another trick it can play: what is often called “rest and digest”, the opposite of the fight or flight response.
Now say you are biking to work when the car next to you swerves to avoid an obstacle and is now encroaching on the bike lane, directly threatening to knock you over. The fight or flight reaction is important in times of emergency like this. However Stephen Porges, PhD, in his breakthrough work, has shown that despite these kind of emergency situations being pretty rare in our lives, many of us are actually in a state of ongoing fight or flight for most of every day. Some of us are even in this state from the moment we wake up to the minute we try to sleep. 
Adapting ourselves to avoid the fight or flight instinct is perhaps the best way to improve the “tone” of our involuntary (aka “autonomic”) nervous system. When we are able to be gentle, the smooth muscle contractions in our brain’s blood supply can function more effectively to remove waste. It also allows our breathing to slow and become more subtle, removing the knots of tension that interfere with the pumping action of breath on our brain’s waste system. We are not talking about deep breaths or even slow breaths. Maybe you already have noticed the natural breathing that you have on waking, but if not then try tomorrow morning — that will give you a taste of the brain-health-promoting nervous system state of rest and digest that we are aiming for here.
As the video explains, improving the tone of our involuntary nervous system just turned out to be crucially important in our efforts to stay healthy and mentally alert as we age. We can thus add brain health as another benefit of gentle, non-striving yoga, meditation and short moments of rest throughout the day.
Far from making us “weak”, softness builds strength and the ability to remain pliable, not just in body but in our brain and mind as we age, adapting quickly when faced with trouble. The science behind this just got a bit more solid.
 Stephen Porges, The Poly-Vagal Theory, http://stephenporges.com
 New Scientist, Best Look Yet at How our Brain’s Sewage System Flushes Out Waste, https://www.newscientist.com/article/mg23130864-200-best-look-yet-at-how-our-brains-sewage-system-flushes-out-waste/