Our brain requires more energy than other organs. However, even when energy supplies are scarce, information processing can be performed flexibly using what is available. How the brain resourcefully allocates this limited energy across internal states remains an important question in neuroscience.
Sleep is an effective way to answer this question. Although sleep involves rest, the brain is still active. This is especially true during rapid eye movement (REM) sleep, a stage closely associated with dreams and memory processing. REM sleep is sometimes called “paradoxical sleep” because the body is mostly still while the brain exhibits wake-like activity. Researchers at Tohoku University have discovered another paradox in REM sleep. The idea is that while the energy supply to the dreaming brain appears to increase, the energy molecules used directly by neurons decrease.
The findings were published in Communications Biology on July 27, 2026.
Have you ever felt exhausted after having a vivid dream? Sleep may seem peaceful, but your brain is extremely active, especially when you’re dreaming. We were intrigued by this contradiction and wanted to find out the scientific basis behind why dreaming is so tiring. ”
Professor Ko Matsui, Tohoku University
To better understand, the researchers used UV-cured resin to make the mouse skulls transparent, allowing them to observe their brains during natural sleep. The researchers used wide-field fluorescence imaging to monitor changes in cerebral blood volume as an indicator of “fuel” supply, neuronal ATP as the energy molecule that powers neurons, and astrocyte pyruvate as a key molecule linking blood-derived glucose to brain energy metabolism.
NREM sleep is best known for strong neuronal activity in the delta band frequencies, but subtle theta band fluctuations are also present. The researchers found that these theta band fluctuations could predict changes in cerebral blood volume seconds later, suggesting that the sleeping brain adjusts vascular dynamics to match ongoing neural activity and metabolic demands.
The transition from NREM sleep to REM sleep showed a different pattern. Approximately 50 seconds before the onset of classically defined REM sleep, blood volume in the brain began to increase. This increase begins in the posterior cortex and spreads anteriorly, suggesting an extensive metabolic preparatory process. After the onset of REM sleep, astrocyte pyruvate also increased. This is consistent with increased substrate availability or increased glycolytic activity of astrocytes. However, paradoxically, neuronal ATP decreased.
The decrease in ATP may be explained by several mechanisms. Neurons can consume large amounts of ATP during REM sleep to support memory-related synaptic reorganization, hippocampal-cortical communication, or large-scale circuit transitions. Alternatively, metabolic communication from astrocytes to neurons may be altered or mitochondrial ATP production may be altered.
This study also illustrates broader principles of biological computation. Unlike traditional computers, animal brains operate under severe metabolic constraints. Rather than distributing energy evenly, the brain may reroute energy depending on behavioral state, memory demands, and 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 is an important function that helps consolidate memories and keep your mind sharp the next day. These findings on energy and sleep are an important step forward in understanding the science behind sleep and how important a good night’s rest is for your mind as well as your body.
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Reference magazines:
Takahashi, Y., others. (2026). The energy paradox in REM sleep: the balance between supply and expenditure in brain metabolism. communication biology. DOI: 10.1038/s42003-026-10646-6. https://www.nature.com/articles/s42003-026-10646-6
