Why do we really need to sleep? A new study from Japan suggests that deep sleep might play an important role in cleaning the brain.
The research, led by Masako Tamaki at the RIKEN Center for Brain Science, found a strong link between deep sleep and cerebrospinal fluid, a clear liquid that surrounds and protects the brain and spinal cord.
The study, published in the journal *Proceedings of the National Academy of Sciences*, looked at how this fluid behaves during different stages of sleep.
Using a quiet form of MRI scanning, the researchers were able to observe changes in cerebrospinal fluid during deep sleep and found that it was closely tied to slow brain waves—electrical patterns in the brain that happen when we’re in deep, non-REM sleep.
We already know that sleep helps store memories and get rid of waste products made by brain activity during the day. But scientists have struggled to figure out exactly how this process works, especially during deep sleep.
Normally, MRI machines are very loud, which makes it hard for people to fall into and stay in deep sleep during experiments. So Tamaki and her team used a special kind of MRI called “sparse functional MRI.”
This method takes scans every few seconds instead of continuously, allowing for quiet periods when people can sleep more deeply. The team also recorded brain waves at the same time to track slow-wave activity.
What they discovered was fascinating. In deep sleep, slow brain waves seemed to cause medium-sized changes in cerebrospinal fluid signals every 8 seconds.
During light sleep or moments when people briefly woke up, the changes in the fluid were less frequent and slower. In REM sleep—the phase when most dreaming happens—the fluid signals were tiny and took almost half a minute to show up.
They also saw that different parts of the brain become active during different sleep stages. In deep sleep, areas related to learning and memory, like the hippocampus and frontal cortex, were more active than during light sleep.
This means deep sleep is not just about resting—it might be the time when the brain clears out waste and supports memory at the same time.
Even though scientists don’t yet fully understand what causes the MRI signal in cerebrospinal fluid—since it doesn’t contain blood like other parts of the brain—the pattern of changes during deep sleep suggests that something important is happening. It could involve cleaning out waste that builds up in memory-related areas while we’re awake.
According to Tamaki, these findings show that deep sleep affects cerebrospinal fluid very differently from other sleep stages. The rapid, yet moderate signal changes might be part of a cleaning system that targets specific types of waste—especially those made in the parts of the brain that help us learn and remember things.
In short, this research helps explain why getting good, deep sleep is so important for brain health. It’s not just about feeling rested—it could also be how our brains stay clean and sharp for the next day.
If you care about sleep, please read studies that Sleeping pill lowers key Alzheimer’s disease markers and Scientists discover the link between sleep apnea and dementia risk.
For more health information, please read studies that Common sleep supplement could improve memory and Scientists find link between short sleep and higher risk of long COVID.
The study is published in PNAS.
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