Turns out dreaming may make your brain stronger

New research from the University of Tsukuba and Kyoto University in Japan is offering up some fresh insight into why dreams are so important. Researchers report capillary blood flow within the brain increases significantly while dreaming, signifying increased neural waste removal and delivery of more oxygen and nutrients. 

Why do we dream?

From a survival perspective, sleep doesn’t make a whole lot of sense. While we sleep we’re susceptible to danger and predators. Nowadays that may not be much of an issue, but it definitely was for our early ancestors. Similarly, the purpose of dreaming is equally confusing. Dreams are usually filled with nonsensical scenarios, people from our past, and a whole lot of surrealism. Dreams can be scary, fun, and sometimes even cathartic, but at the end of the day (or evening?) they usually leave us scratching our heads in confusion more than anything else.

All of this begs the nagging question: What is the purpose of dreaming and sleeping? It must be important, or else evolution and nature wouldn’t demand that the vast majority of animals on planet earth get at least a little shuteye every 24 hours.

Modern medicine tells us that sleep is a time for the brain to reset, recharge, and process newly formed memories. But, what about dreams? If the mind is supposed to be relaxing while we sleep, why is it constantly conjuring up weird dream scenarios like returning to high school with unfinished homework?

This latest research can’t answer all our questions about sleep and dreams, but it’s a step in the right direction. It’s still unclear exactly why dreams occur while increased levels of red blood cells clean out and refresh our minds. However, this work does strongly indicate that dreaming is essential to a truly neurologically restful night’s sleep.

The vast majority of dreams occur during the rapid eye movement (REM) phase of sleep, and while prior research projects have assessed blood flow levels between various sleep stages, they’ve all produced largely conflicting findings.

What happens to your brain when you sleep?

So, in an effort to gain some clarity, researchers used a groundbreaking new technique on a group of lab mice. This process allowed study authors to watch in real-time the flow of blood throughout the rodents’ brains as they passed through various sleep stages.

“We used a dye to make the brain blood vessels visible under fluorescent light, using a technique known as two-photon microscopy,” explains senior study author of the study Professor Yu Hayashi. “In this way, we could directly observe the red blood cells in capillaries of the neocortex in non-anesthetized mice.”

Meanwhile, individual sleep stages were identified by measuring electrical activity in the mice’s brains during sleep as well. Upon getting started, even the researchers themselves were shocked by what they saw.

“We were surprised by the results,” Professor Hayashi says. “There was a massive flow of red blood cells through the brain capillaries during REM sleep, but no difference between non-REM sleep and the awake state, showing that REM sleep is a unique state.”

At one point the research team woke up the mice during REM sleep. Interestingly, when the mice fell back asleep that disruption caused “rebound REM,” or a stronger version of REM sleep to make up for the lost time. Sure enough, blood flow throughout the rodents’ brains increased even more during this “rebound” phase. Study authors theorize this suggests a direct relationship between REM sleep intensity and neural blood flow levels/brain refreshment.

That same process was recreated with another group of mice, except this cohort was lacking adenosine A2a receptors. For reference, whenever coffee is consumed it results in the blockage of adenosine A2a receptors, which is partially why coffee helps us wake up. Those mice didn’t display nearly as much blood flow during REM sleep or rebound REM sleep.

“These results suggest that adenosine A2a receptors may be responsible for at least some of the changes in blood flow in the brain during REM sleep,” Professor Hayashi concludes.

Both diminished blood flow in the brain and lack of REM sleep are associated with dementia and Alzheimer’s onset. Moreover, dementia is linked with the buildup of neural waste and proteins in the mind. All of those factors, combined with these new findings, certainly make a strong case for REM sleep and dreaming as an essential part of keeping the mind sharp well into old age. 

Study authors believe much more research on adenosine A2a receptors in particular is warranted, and may one day lead to new dementia treatments and prevention strategies.

The full study can be found here, published in Cell Reports.