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Brain refreshing: Why the dreaming phase matters

Previous studies have measured differences in blood flow in the brain between REM sleep, non-REM sleep, and wakefulness using various methods, with conflicting results. In their latest work, the Tsukuba-led team used a technique to directly visualize the movement of red blood cells in the brain capillaries (where nutrients and waste products are exchanged between brain cells and blood) of mice during awake and asleep states.

"We used a dye to make the brain blood vessels visible under fluorescent light, using a technique known as two-photon microscopy," says senior 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."

The researchers also measured electrical activity in the brain to identify REM sleep, non-REM sleep, and wakefulness, and looked for differences in blood flow between these phases.

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

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