Sleep offers a useful way to study this question. Although the body is at rest, the brain remains active—particularly during rapid eye movement (REM) sleep, the stage closely associated with vivid dreams and memory processing. REM sleep is often called “paradoxical sleep” because brain activity resembles wakefulness while the body remains mostly still.

Researchers at Tohoku University have identified another REM sleep paradox: during this stage, the apparent supply of energy to the dreaming brain increases, while levels of the energy molecule used directly by neurons decrease. The findings were published in Communications Biology.

“Have you ever felt exhausted after having a vivid dream?” asks Ko Matsui, a professor at Tohoku University. “Sleep may seem restful, but the brain is highly active, especially when dreaming. We were interested in this contradiction and wanted to investigate the scientific basis behind why dreaming is tiring.”

How researchers observed the sleeping brain in real time

To investigate how brain energy changes during sleep, the researchers used UV-cured resin to make the skulls of mice transparent. This allowed them to observe the animals’ brains during natural sleep.

Using wide-field fluorescence imaging, the team tracked changes in cerebral blood volume as an indicator of incoming fuel. They also measured neuronal ATP, the energy molecule that powers neurons, along with astrocyte pyruvate, a key compound linking glucose from the blood to brain energy metabolism.

Non-REM (NREM) sleep is known for strong neuronal activity at delta-band frequencies, but smaller fluctuations in the theta band also occur. The researchers found that these theta changes could predict shifts in cerebral blood volume several seconds later.

This pattern suggests that during sleep, the brain adjusts its blood vessels in response to changing neuronal activity and metabolic demands.

The brain prepares for REM sleep before it begins

When the brain transitioned from NREM sleep to REM sleep, the researchers observed a different pattern.

About 50 seconds before the onset of classically defined REM sleep, blood volume in the brain began to rise. The change started in the posterior cortex and then moved forward, suggesting a large-scale process that may metabolically prepare the brain for REM sleep.

Pyruvate levels in astrocytes also increased as REM sleep began. This rise was consistent with greater metabolic fuel availability or increased glycolytic activity in astrocytes.

However, neuronal ATP decreased at the same time.

Why might neuronal energy drop during dreams?

The researchers identified several possible explanations for the decrease in ATP.

Neurons may use substantial amounts of ATP during REM sleep to support memory-related synaptic reorganization, communication between the hippocampus and cortex, or widespread changes across brain circuits.

Another possibility is that the transfer of metabolic resources from astrocytes to neurons changes during REM sleep. Mitochondrial ATP production may also be altered during this stage.

Whatever the mechanism, the findings suggest that the dreaming brain may operate under unusually high energy demands despite receiving an increased supply of fuel.

What REM sleep reveals about the brain’s energy economy

The results may reveal something more fundamental about biological computation.

Unlike traditional computers, animal brains must function within strict metabolic limits. Instead of distributing energy evenly, the nervous system may redirect resources according to behavioral state, memory demands, and other internal needs.

“Understanding how the brain balances energy supply and expenditure may help explain why biological intelligence is so efficient,” explains lead researcher Yusuke Takahashi. “REM sleep provides a natural example of how the brain reorganizes its energy economy to support complex internal processing.”

Sleep contributes to functions such as memory consolidation and maintaining mental performance the following day. By showing how energy supply and expenditure change during REM sleep, this study adds another piece to the puzzle of why sleep is important—not only for the body, but also for the brain.