Brain energy dips during REM sleep despite increased blood flow
Researchers at Tohoku University studied sleeping mice by making their skulls transparent and tracking brain activity with fluorescence imaging. They observed that blood supply to the brain increased before REM sleep began, yet levels of ATP, the energy molecule neurons use directly, fell during REM. The results suggest that dreaming involves such intense internal processing that neurons consume energy faster than it can be replaced.
Using a UV-curable resin, Tohoku University scientists rendered mice skulls see-through, then applied wide-field fluorescence imaging to monitor cerebral blood volume, neuronal ATP, and astrocytic pyruvate during natural slumber. In non-REM sleep, theta-band fluctuations forecast blood-volume shifts seconds later, hinting that vessels respond to changing neural demand.
Roughly 50 seconds before REM onset, blood volume climbed, spreading from the back of the cortex toward the front. Once REM arrived, astrocytic pyruvate increased while neuronal ATP dropped. Possible explanations include intense memory-related synaptic remodeling and hippocampal communication draining ATP faster than it is replaced.
This finding may shift how people interpret sleep as purely restorative, especially those who wake exhausted after vivid dreams or have REM-related sleep disturbances. It could encourage clinicians and researchers to consider brain energy dynamics alongside blood flow when studying sleep quality, memory, and fatigue. For now, the work is in mice, so any implications for human dreaming, cognition, or sleep medicine remain speculative and would need further study.