The Brain Shows a Coordinated Shift in Activity as We Fall Asleep

A new study by investigators from Mass General Brigham used next-generation imaging technology to discover that when the brain is falling asleep, it shows a coordinated shift in activity. They found that during non-rapid eye movement (NREM) sleep, parts of the brain that handle movement and sensory input stay active and keep using energy, while areas involved in thinking, memory and daydreaming quiet down and use less energy. Their results are published in Nature Communications.

“This research helps explain how the brain stays responsive to the outside world even as awareness fades during sleep,” said corresponding author Jingyuan Chen, PhD, an assistant investigator at the Athinoula A. Martinos Center for Biomedical Imaging at Massachusetts General Hospital, a founding member of the Mass General Brigham healthcare system. “By revealing how brain activity, energy use, and blood flow interact during sleep, these findings, and the imaging tools we used to uncover them, offer new insights into the mechanisms of neurological and sleep-related diseases.”

The body cycles through two types of sleep several times each night: NREM and rapid eye movement (REM). NREM is the deep, restorative stage of sleep that plays a key role in physical health, brain function and disease prevention. Yet, many of its underlying processes and impacts on long-term health remain poorly understood. Previous studies have suggested that NREM helps clear waste from the brain.

Using a new tri-modal EEG-PET-MRI technique that combines EEG to study brain activity, functional MRI to analyze blood flow, and functional PET (fPET)-FDG to monitor glucose metabolic dynamics, researchers examined the brains of 23 healthy adults during brief afternoon sleep sessions.

The researchers found that energy use and metabolism decrease as sleep deepens, while blood flow becomes more dynamic, especially in sensory areas that stay relatively active. At the same time, higher-order cognitive networks quiet down, and cerebrospinal fluid flow increases. Together, these findings support the idea that sleep helps clear waste from the brain while maintaining sensitivity to sensory cues that can trigger awakening.

The authors note future studies should include larger, more diverse groups and collect longer, deeper sleep recordings. The researchers also plan to use more precise methods to measure brain metabolism and better distinguish between sleep stages.

Reference: Chen JE, Lewis LD, Coursey SE, et al. Simultaneous EEG-PET-MRI identifies temporally coupled and spatially structured brain dynamics across wakefulness and NREM sleep. Nat Commun. 2025;16(1):8887. doi:10.1038/s41467-025-64414-x

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