A new study shows that sleep is an important cleaning process that removes toxic products from the brain. The results of the study were published in the journal Science.
We spend almost a third of our lives sleeping. But the reason why we sleep remains a mystery. Scientists have never agreed conclusively on a certain explanation. But recent studies show that sleep might be a way for the brain to clean itself.
An inevitable phenomenon of biological activity is the generation of waste products. It happens everywhere in our body. That’s why we have the lymphatic system, a network of organs and vessels which filters and removes the waste throughout our body, but not the brain. The brain has its own cleaning system, the cerebrospinal fluid (CSF). CSF is found in the space between cells and tissues (interstitial space) of the brain and its flow is controlled by pulse waves originating from the brain arteries. CSF protects the brain from knocks but also provides nutrients and filters waste products which harm the nerve cells and they cannot communicate properly.
Scientists knew that the amount of β-Amyloid, a waste product, in the brain is much less during the sleeping state. Maiken Nedergaard, a medical doctor and sleep researcher who led the study, and her team collaborators hypothesized that this fluctuations might have something to do with the wakefulness.
So, they designed an experiment to measure the flow of CSF into the cortex of the sleeping, anesthetized and awake mice. They first connected the brains of each mouse to specialized devices which track CSF flow. Then, when the mouse was asleep, they saw big amount of CSF circulating around arteries and in between brain regions. They later fondled the tail to wake up the mouse and they saw the CSF disappearing from the aformentioned brain regions. The reverse was seen when they started with an awake mouse which was later anesthetized by an injection. The increase in CSF amounts in the interstitial space of the brain was 95% in the sleeping or anesthetized mice when compared to the awake mice.
By measuring the size of the interstitial space, they found that it is contracted, and therefore smaller in conscious mice inhibiting the influx of fluids. But how is the size of the interstitial space controlled? The scientists know that when we regain consciousness after sleep, certain neurotransmitters (molecules that modulate neurons) allow the brain to recover and regain fast rhythms; this is called the ascending arousal system. When they blocked these molecules in the awake mice, they saw that the interstitial space is getting larger and the CSF is flowing back in the brain, similar to the flow of sleeping mice.
This flow of CSF in the brain had a beneficial result. Several waste products, such as β-Amyloid and others, were removed from the brain of sleeping (or anesthetized) mice.
So, what it seems to be happening is that sleep switches on a state in the brain during which a cleaning process takes place. All waste products produced in the brain during the awake state are washed away by the increased flow of fluids that are allowed in the interstitial space only during sleep.
Lack of sleep, which is associated with neuropathologies such as insomnia, narcolepsy, Alzheimer’s and Parkinson’s diseases, affects more than 40 million individuals in the U.S. Sleep deprivation can reduce learning ability and cognitive functions and might even cause death if prolonged. In addition, the study “could open a lot of debate for shift workers, who work during the nighttime,” says Nedergaard. “You probably develop damage if you don’t get your sleep.”