Experiment did not support the hypothesis of brain cleansing in sleep

It is believed that one of the main functions of sleep is to actively cleanse the brain of metabolites and toxins. However, a recent experiment with mice has cast doubt on this common scientific explanation.

It's no secret that a sound night's sleep helps recovery, while sleep deprivation is fraught with poor health, impaired cognitive function, hand-eye coordination, and so on. Therefore, the hypothesis that during sleep in the brain is a kind of "cleaning", seems logical. However, the assumption is based on indirect evidence, and direct evidence that in sleep in the brain activates the removal of waste, scientists have not yet.

A team of physicians from Imperial College London (UK) took on the task. In experiments with laboratory mice, they used fluorescent dye to track what happens in the brain of animals during sleep, wakefulness and under anaesthesia.

The experiment allowed them to see how quickly the dye moved from the so-called lateral ventricles of the brain - cerebrospinal fluid-filled cavities - to other areas, as well as measure the rate at which the dye was excreted directly from the brain.

In the end, it was found that in sleeping mice and subjects under anaesthesia, brain clearance was about 30% and 50% slower than in awake animals. The results were published in the journal Nature Neuroscience.

"[Past] research is so centred around the idea of clearing as one of the key reasons why people sleep, and we were of course very surprised to see the opposite in the experimental results. We found a significant reduction in the rate of dye excretion from the brain in animals that were asleep or anaesthetised," Professor Nick Franks, one of the study's authors and an expert in biophysics and anaesthetics from Imperial College London, told The Guardian in a commentary.

The scientists believe their findings also apply to humans, as sleep is a basic physiological requirement of the body in all mammals.

There are many assumptions about exactly why sleep is needed. While the new study suggests that the key reason may not be related to the removal of toxins from the brain, the importance of sleep remains undeniable.

The results of the experiment could be valuable in understanding the causes of dementia and Alzheimer's disease - medics have growing evidence linking these disorders to poor sleep. It is not yet clear whether lack of sleep contributes to the development of Alzheimer's disease or is a consequence of it. Some researchers believed that sleep deprivation could impair the process of cleansing the brain of toxins. However, in light of the UK findings, this explanation now seems less convincing.

However, the authors noted another important point: the experiment showed that the brain is effectively cleansed during wakefulness. This means that wakefulness, physical activity and exercise may help to improve metabolism and remove toxins from the brain. This conclusion may have serious practical implications: it turns out that high activity in normal conditions allows the brain to rest even better than in sleep.

Previously, a team of experts from the US, Canada and Australia found that reducing deep sleep by 1% increases the risk of dementia by 27%. In another study, scientists found a link between daytime sleep and the development of Alzheimer's disease. It turned out that excessively long sleep during the day can lead to the progression of senile dementia.