Scientists find that the brain’s energy supply rises before REM sleep, while the energy available to neurons falls. Dreaming may feel like the brain’s version of doing nothing, but new research suggests it could be quite the opposite.

Scientists have found that during rapid eye movement (REM) sleep, the stage most closely associated with vivid dreaming, blood supply to the brain increases while the amount of immediately available energy inside neurons falls.

The findings, published in Communications Biology, could offer new clues about how the brain manages its energy during sleep.

“Ever felt exhausted after a vivid dream?” said Professor Ko Matsui of Tohoku University. “Sleep may appear peaceful, but the brain is highly active — especially when dreaming.”

REM sleep is sometimes called “paradoxical sleep” because the body is largely still while brain activity resembles that seen during wakefulness.

Researchers at Tohoku University wanted to understand how the brain manages its energy during this unusually active state.

Watching the sleeping brain

The researchers studied sleeping mice using a technique that allowed them to observe the brain through a transparent skull.

They used wide-field fluorescence imaging to track changes in brain blood volume, which provided an indication of the supply of nutrients and oxygen reaching the brain.

They also measured levels of adenosine triphosphate (ATP), the molecule neurons use as an immediate source of energy, as well as pyruvate in astrocytes, cells that support neurons and play an important role in brain metabolism.

The researchers found that changes in neuronal activity during non-REM sleep could predict changes in brain blood volume several seconds later.

This suggests that blood vessels in the sleeping brain adjust their activity in response to changing neuronal activity and energy demands.

The brain prepares for dreaming

The most striking changes appeared as the mice moved from non-REM sleep into REM sleep.

Brain blood volume began increasing around 50 seconds before the researchers identified the start of REM sleep.

The increase began in the posterior cortex before spreading towards the front of the brain, suggesting that the brain may begin preparing for the increased demands of REM sleep before the state actually begins.

Once REM sleep started, levels of pyruvate in astrocytes also increased.

That could indicate that more metabolic fuel was becoming available or that astrocytes were increasing their glycolytic activity.

But there was a surprising twist: neuronal ATP levels fell.

More fuel, less energy

The researchers say there are several possible explanations for the drop in ATP.

REM sleep may place particularly high energy demands on neurons as the brain reorganises connections involved in memory and communication between different regions.

Another possibility is that the way metabolic resources move between astrocytes and neurons changes during REM sleep. Changes in mitochondrial activity could also affect how efficiently neurons produce ATP.

The study does not establish exactly why ATP levels fall, but it suggests that the brain may be consuming energy faster than it can immediately replenish it during REM sleep.

In other words, the brain appears to be increasing its fuel supply while simultaneously burning through its readily available energy.

A closer look at the brain’s energy budget

The findings could also shed light on a broader question: how does the brain power complex activity while operating within strict metabolic limits?

Unlike conventional computers, the brain cannot simply draw unlimited energy when processing more information. It has to constantly balance energy supply and demand.

“Understanding how the brain balances energy supply and consumption may help explain what makes biological intelligence so efficient,” said lead investigator Yusuke Takahashi.

REM sleep provides a natural example of that balancing act, showing how the brain can reorganise its energy use while carrying out complex internal processes.

Sleep is already known to play an important role in memory consolidation and maintaining cognitive performance. The new findings suggest that understanding the brain’s energy use during different stages of sleep could provide another piece of the puzzle of why sleep matters so much.

So while the rest of the body may be lying completely still, the brain may be working overtime — and dreaming could be one of its more energy-hungry shifts.