Researchers at Tohoku University used a transparent-skull technique in mice to watch brain energy metabolism during natural sleep. They tracked blood volume as a proxy for fuel delivery, along with neuronal ATP and astrocytic pyruvate, a molecule linking blood glucose to brain energy use.
The team found that blood volume started climbing in the posterior cortex about 50 seconds before REM began, as if the brain were preparing for the upcoming state. Once REM started, pyruvate rose too, yet neuronal ATP declined. The authors describe this as a paradox: the dreaming brain appears to receive more fuel while its neurons' immediately usable energy store shrinks.
Possible explanations include heavy ATP consumption for memory-related synaptic reorganization, communication between the hippocampus and cortex, or changes in how mitochondria and astrocytes supply energy to neurons. The researchers also note that biological brains, unlike conventional computers, must operate within strict metabolic limits, and REM sleep offers a natural example of how the brain shifts its energy economy to meet internal demands.