Researchers at Tohoku University monitored the brains of naturally sleeping mice using specialized imaging. Blood flow increased markedly from the back toward the front of the brain about 50 seconds before REM sleep began.
Pyruvate, a precursor used in cellular energy production, also rose, suggesting that the brain increases its fuel supply ahead of this sleep phase, when dreams are often vivid.
Yet levels of ATP—the main energy source used directly by neurons—dropped sharply. The findings indicate that during intense dreaming, neurons may consume energy faster than it is replenished.
Study leader Professor Ko Matsui said the team wanted to understand why people can feel tired after a vivid dream despite sleep appearing calm from the outside. The researchers point to memory consolidation, synaptic reorganization and intensive information exchange between the hippocampus and cortex as possible contributors to the energy demand. The work may help explain how the brain manages limited metabolic resources and how sleep affects memory.