Scientists from Mass General Brigham have discovered how the brain undergoes a perfectly synchronized shift in activity as it transitions from wakefulness tosleep. Published inNature Communications,the study found that while the brain’s sensory and motor regions stay active during non-rapid eye movement (NREM) sleep, higher cognitive areas responsible for thinking and memory reduce activity and energy use. This dynamic balance helps the brain remain partly responsive to the external world, even while consciousness fades. NREM sleep is a deep, restorative phase crucial for physical recovery, brain maintenance, and memory consolidation. It is also thought to help flush out metabolic waste from the brain. However, the interactions between energy use,brain activity, and blood flow during this stage have remained unclear. The study sought to reveal these hidden mechanisms that sustain both rest and responsiveness during sleep. For this study, researchers recruited 23 healthy adult volunteers for short, afternoon nap sessions. Using an advanced tri-modal imaging method-EEG-PET-MRI-they simultaneously measured three aspects of the brain: electrical activity (EEG), blood flow (fMRI), and glucose metabolism (functional PET-FDG). This powerful approach allowed scientists to track how neuronal, vascular, and metabolic systems interacted in real time as participants entered and deepened into NREM sleep. The results showed that overallbrain metabolismdecreased as sleep deepened, yet blood flow in sensory areas became more dynamic. In contrast, higher-order networks—linked to cognition and daydreaming—quieted down, and cerebrospinal fluid circulation increased. These patterns support the idea that the sleeping brain continues performing vital “housekeeping” functions while maintaining sensitivity to meaningful sensory triggers. Lead author Dr. Jingyuan Chen noted that this research offers deeper insights into how brain activity and energy regulation during sleep may relate to neurodegenerative and sleep-related diseases. The team plans to expand this work with larger, more diverse samples and longer sleep recordings to refine their understanding of the brain’s complex choreography during rest. Reference:Chen, J. E., et al. (2025). Simultaneous EEG-PET-MRI identifies temporally coupled and spatially structured brain dynamics across wakefulness and NREM sleep. Nature Communications. doi.org/10.1038/s41467-025-64414-x