This Is Why The 'Dream Phase' Of Sleep Matters, Researchers Find 'Surprising' Evidence

Why is it so important for people to reach the rapid eye movement (REM) phase of sleep? A team of researchers had a closer look at the possible "brain refreshing" function that happens during this "dream phase."

Sleep is an important part of people's routines, and even animals do it. Quality sleep is "as essential to survival as food and water" as it affects how we function in ways that scientists are still only beginning to understand, the National Institutes of Health (NIH) said.

Basically, the two types of sleep are non-REM sleep and REM sleep. REM sleep happens about an hour after falling asleep, and it's when people have vivid dreams.

"Sleep is generally viewed as a period of recovery, but how the supply of cerebral blood flow (CBF) changes across sleep/wake states has remained unclear," the researchers of a study, published in Cell Reports, said.

Previous studies that looked at the differences in blood flow during REM, non-REM and wakefulness have had "conflicting results," the University of Tsukuba said in a news release. For their study, the researchers used a method that allowed them to actually observe the red blood cells in the capillaries.

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

The researchers also measured electrical activity to detect non-REM, REM and wakefulness, the university noted.

"We were surprised by the results," Professor Hayashi said. "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."

Capillary blood flow in the brain is important for delivering nutrients and oxygen, as well as for removing waste products. And when the researchers disrupted the mice's sleep, they observed a further increase in cerebral blood flow, suggesting a link between the blood flow and REM sleep strength.

But when they repeated the experiments in mice without adenosine A2a receptors, which help make one feel more awake after coffee consumption, the researchers observed that there was less increase in the blood flow during REM sleep.

"At the molecular level, signaling via adenosine A2a receptors is crucial; in A2a-KO mice, capillary CBF upsurge during REM sleep is dampened, and effects of REM sleep pressure are abolished." the researchers wrote.

The graphical abstract of the findings showed the differences in the blood flow that the researchers observed.

"Given that reduced blood flow in the brain and decreased REM sleep are correlated with the development of Alzheimer's disease, which involves the buildup of waste products in the brain, it may be interesting to address whether increased blood flow in the brain capillaries during REM sleep is important for waste removal from the brain," the University of Tsukuba said.