Recent research shows that a major function for sleep may be to facilitate the brain’s janitorial functions. Waste products and the detritus of daily thinking can be removed during nightly sleep. This cleaning function allows our brains to work properly and may account for why we generally feel brighter and happier in the mornings.
Other organs and tissues are "cleaned" using the lymphatic system that almost works as a sewage system for the body. The lymphatic system does not extend into the brain. The so-called "blood-brain barrier" limits fluid channel exchanges between the brain and the rest of the body to protect the brain.
The extracellular fluid in the brain – the wetness between the neurons – is part of a system that essentially flushes the brain and removes waste. This has been named the glymphatic system. It was previously thought that waste products were metabolized and broken down by the neurons themselves and the results of that breakdown carried away in capillaries and ultimately removed by the kidneys.
You may have heard of the brain protein beta-amyloid, present in high levels of brains with Alzheimer’s Disease. How does a healthy body without Alzheimer’s remove the beta-amyloid? With this new model, that protein, and other waste products are washed away by the glymphatic system.
The interstitial space makes up about 20% of the brain volume but that fraction varies over the course of the day and night. During sleep this space increases by up to 60% in volume.
This gives us a hint as to what the restorative function of sleep may be due to, or at least part of the restorative function. This flushing of the glymphatic system removed waste metabolic products that are potentially neurotoxins. These products include β-amyloid proteins which are strongly suspected to play a part in Alzheimer's DIsease.
We’ve long known that being sleepy reduces your mental capacity and ability to acquire new skills and knowledge. Sleep deprivation reduces cognitive capacity. But why?
Experiments in mice show that at a microscopic brain cell level, sleepiness and sleep debt affect the brain. Nerve cell dendrites - the extension that helps connect a nerve cell to other nerve cells - grew at a slower rate in sleepy mice. The rate of growth was correlated to how well the mice learned new tasks.
Even in waking mice, the rate of growth was higher when the animal had sufficient sleep compared to the rate in sleep-deprived animals. In normal sleep/wake cycles outside a heaving learning environment, dendrites tend to disappear during sleep and form during waking. Also, the researchers could see the dendrites grow when the mice learned new tasks. In the slow-wave cycle of the sleep period that followed, these new dendrites "fired" again, leading strong evidence to the hypothesis that this growth is critical in the formation of new memories.
If this new theory turns out to be true, it could mean recovery sleep is not as efficient as previously thought. We have stated at Sleepdex that one or two nights of recovery sleep is enough to get over any sleep deprivation.
The appealing aspect of this hypothesis is - in addition to being backed by laboratory evidence - that it helps explain sleep's role in both memory formation and in the general restorative function on the brain and why sleep deprivation reduces cognitive capacity. It akso might be another reason for the evolution of sleep.
The hypothesis may partly explain why larger animals sleep less than smaller ones. The larger brains have more interstitial space and can remain awake longer before the cleaning function of sleep is required.
The glymphatic system is composed of glial cells (a type of cell common in the nervous system) and in particular a type of cell called astrocytes.
It's long been known that cerebrospinal fluid goes between the head and the rest of the body. Scientists even examine the cerebrospinal fluid for clues to what is happening in the brain. In the glymphatic system hypothesis, this fluid is the very medium of the brain washing.
This raises the possibility that a faulty glymphatic system causes - or at least increases the risk for - Alzheimer's.
Does this mean that chronic sleep deprivation can increase the risk of dementia? Nobody is saying that yet, but it certainly suggests a possible connection. One more reason to get plenty of sleep.