A study conducted at the University of Tsukuba has reported that cerebral blood flow increases during the Random Eye Movement (REM) stage of sleep. This finding shows that the removal of wastes from the brain during REM sleep refreshes the brain.

Researchers published the study Cerebral Capillary Blood Flow Upsurge During REM Sleep is Mediated by A2a receptors in Cell Reports.

How researchers conducted the study

The researchers used two-photon microscopy, where they used a dye to examine the capillary blood flow of mice under fluorescent lighting. According to Professor Yu Hayashi, a lead study author, the technique enabled them to see red blood cells in capillaries of the neocortex. In addition, they noted that blood flow rose during REM sleep but remained constant when the mice were aware and during non-REM sleep.

Afterward, the researchers interrupted the mice’s sleep causing rebound REM sleep, a more robust version of REM sleep that compensates for lost sleep. During this rebound phase, capillary blood flow was even higher, showing that the strength or REM sleep could influence blood flow.

When they repeated these experiments in mice with no adenosine A2a receptors, blood flow during both REM sleep and rebound REM sleep did not increase markedly.

As the function of capillaries is to supply nutrients and oxygen while eliminating wastes, this could explain how REM sleep refreshes the brain. Furthermore, researchers concluded that adenosine A2a receptors might play a role in the increased blood flow.

This study is not the first to investigate blood flow between REM, non-REM sleep, and wakefulness. However, most of these studies present conflicting results because of the methods used. With two-photon microscopy, Hayashi and his team were able to view capillaries more directly. They also measured the brain’s electrical activities during these different stages.

Scientists could use these findings to develop a treatment for Alzheimer’s disease

The team hopes that the role of adenosine A2a receptors could help other scientists come up with more effective treatments for Alzheimer’s disease. This disease is associated with insufficient blood flow to the brain and less REM sleep which could cause wastes to accumulate in the brain.