“Why do we sleep? To clean our brains, say US scientists,” The Guardian reports. A US research team, studying mice, has suggested sleep helps clear the brain of 'waste products’.
Although sleep is an intrinsic part of our lives, scientists are still uncertain why a good night’s sleep makes us feel better or why lack of sleep is bad for us.
Now a US research team claims to have found at least part of the answer. The study looked at whether sleep influences how the brain removes its waste products.
Waste products produced by the cells in the brain are deposited into the fluid surrounding the cells. One way that these are then removed is through movement into the cerebrospinal fluid (CSF) surrounding the brain and spinal cord. The study looked at whether this process was influenced by sleep in mice.
It found that when the mice slept, the amount of space around the brain cells increased. This caused more fluid to move from the CSF into these spaces. This seemed to result in a quicker clearance of waste products, such as amyloid beta – a protein which builds up in the brains of people with Alzheimer’s.
Brain imaging studies in humans will be needed to confirm whether a similar thing happens to us, but it does seem likely.
This study provides further clues about why sleep is essential. Read more about the effects of sleep on health.
Where did the story come from?
The study was carried out by researchers from the University of Rochester and New York University in the US. It was funded by the National Institutes of Health and the National Institute of Neurological Disorders and Stroke.
The study was published in the peer-reviewed journal Science.
The BBC News website gives a good explanation of this study and the limitations of what can be said about the findings.
The Guardian’s reporting of the study was also of a good quality. It included quotes from UK researchers working on sleep who sounded a note of caution about the need for the results to be independently confirmed. For example, Dr Vladyslav Vyazovskiy of Surrey University was quoted as saying “I'm not fully convinced. Some of the effects are so striking they are hard to believe. I would like to see this work replicated independently before it can be taken seriously”.
Other UK sources reported the study accurately but with less background detail or context.
What kind of research was this?
This was animal research that aimed to understand the function of sleep. The researchers report that although a lot of research has been done on sleep, it is still not understood why it has restorative effects, or why our brain doesn’t function well without it. Continuous sleep deprivation can be fatal.
The researchers wanted to test the possibility that sleep helps to clear the brain of potentially damaging chemicals, in particular the amyloid beta protein which builds up in Alzheimer’s disease. This protein, others linked to degenerative brain diseases, and other waste products from the nerve cells are found in the fluid surrounding these cells in the brain, called the “interstitial fluid”. They are removed from the brain in part through the fluid circulating around the brain and spinal cord, called the “cerebrospinal fluid” or CSF. CSF is a fluid that surrounds, supports and protects the brain and spinal cord.
The CSF and interstitial fluid are separated by a membrane barrier. Fluid and certain substances can either naturally move across the membrane or be ‘pumped’ across the membrane.
What did the research involve?
The researchers used fluorescent imaging techniques to monitor how fluid from the CSF moves around the brain in awake and asleep mice. They injected a fluorescent dye into the CSF and then followed how it moved between the CSF and interstitial fluid when the mouse was asleep or awake. They also monitored brain activity at the same time so they could accurately detect when the mice were asleep. They repeated their experiments to look at what happened in anaesthetised mice.
The researchers also carried out experiments to:
- inject the mice’s brains with radioactively tagged amyloid beta and follow how quickly it was removed into the CSF when the mice were asleep, anaesthetised, or awake
- measure the volume of interstitial fluid (and therefore the interstitial space itself) in the brain while the mice were asleep, anaesthetised, or awake
- assess how any changes in interstitial space volume might come about
What were the basic results?
The researchers found that when the mice were asleep, a considerable amount of the dye moved out from the CSF and into the interstitial space. When the mice were awake, significantly less (about 95% less) of the dye moved between the CSF and interstitial fluid. The researchers found that anaesthetising the mice had a similar effect on dye movement as sleep.
The mice were also found to clear amyloid beta from their brains more quickly while they were asleep or anaesthetised than when they were awake.
The researchers found that the volume of the interstitial fluid (and therefore the interstitial space it fills) increased by more than 60% when asleep or anesthetised compared to when awake.
This difference suggested that a contraction of the interstitial space when the mice were awake could be contributing to the reduction in the movement of dye (and fluid) between the CSF and interstitial fluid.
They found that a particular type of chemical signalling in the brain, called adrenergic signalling was involved in causing the changes in interstitial space volume seen.
How did the researchers interpret the results?
The researchers concluded that sleep may be restorative because it increases the removal of potentially toxic waste chemicals that accumulate in the brain during waking.
This fascinating research has suggested that sleep helps potentially toxic substances to be removed from the brain. How sleep restores us and helps our brains to function is reported to not be well understood. Although this research was in mice rather than humans, despite the difference in size, this type of basic biological function is seems to be very similar to what happens in all vertebrate animal species. Further research in humans will help to confirm this.
As yet, whether there could be a link between changes in this normal function in the removal of amyloid beta or other potentially damaging substances and brain diseases such as Alzheimer’s disease is not known. Undoubtedly this is an avenue of research that researchers will consider worth exploring.
Overall, this study provides fresh insights into the purpose of sleep. It will be interesting to see if the results can be replicated by other researchers, and in humans. Sleep is essential to our health, and it seems likely that there will be multiple reasons why this is the case, rather than a single answer.