Researchers at Mass General Brigham have charted the brain’s activities during the exact moments we fall asleep, unveiling a surprisingly synchronized sequence of events. By employing a cutting-edge imaging method that simultaneously captures three distinct brain signals, scientists observed that instead of merely shutting down, sleeping brains executed a meticulously coordinated transition, with some areas remaining active while others quieted down.
The investigation concentrated on NREM (non-rapid eye movement) sleep, the profound, dreamless phase that cycles throughout the night before REM sleep begins. Although it has been well-established that NREM sleep is vital for health and memory, the intricate processes governing the brain’s shift into this state have remained quite elusive. Twenty-three healthy participants volunteered to take naps inside a scanner during afternoon sessions, with their brain activity monitored via a tri-modal setup that integrated EEG for electrical signals, fMRI for blood circulation, and functional PET imaging to observe glucose metabolism.
The results from the scans were remarkable. As volunteers transitioned into NREM sleep, the brain’s sensory and motor regions—the parts that interpret touch, sound, and motion—remained unexpectedly active and continued to utilize energy. Conversely, higher-order areas involved in complex thought processes, memory consolidation, and the mind-wandering default mode drastically reduced their activity, consuming significantly less glucose.
Why Certain Brain Areas Remain Engaged
This trend indicates that the sleeping brain upholds a kind of monitoring system. Even during deep sleep, sensory regions must remain partially functional to identify possible threats: a smoke detector, a crying infant, an unfamiliar sound. This discovery sheds light on a longstanding inquiry regarding how we stay alert to our surroundings even when our awareness diminishes.
“This research elucidates how the brain remains attuned to the external environment even as consciousness diminishes during sleep.”
Blood flow patterns introduced an additional dimension to the findings. Instead of decreasing uniformly, blood circulation turned more dynamic during NREM sleep, especially in the sensory areas that remained active. Simultaneously, cerebrospinal fluid, the transparent liquid that envelops the brain and spinal cord, increased its flow. This observation supports recent hypotheses suggesting that sleep performs a housekeeping role, effectively removing the brain’s metabolic waste while we are unconscious.
The imaging technology itself signifies a notable technical accomplishment. Jingyuan Chen, who headed the research at Massachusetts General Hospital’s Martinos Center for Biomedical Imaging, remarked that the integration of three imaging techniques in real-time provides unprecedented insights into how activity, metabolism, and blood circulation are interconnected.
“By illustrating how brain activity, energy consumption, and blood flow interplay during sleep, these discoveries, along with the imaging tools implemented, provide new understandings of the mechanisms underlying neurological and sleep-related disorders.”
Broader Implications Beyond Normal Sleep
The findings may ultimately enhance our comprehension of sleep disorders and neurological diseases. If NREM sleep aids in waste removal, as suggested by this and other studies, interruptions to these coordinated mechanisms could contribute to conditions such as Alzheimer’s disease, where protein accumulation in the brain plays a crucial role. This research also prompts inquiries about what occurs when this delicate equilibrium is disrupted, whether due to sleep deprivation, aging, or ailments.
The researchers acknowledged some constraints in their study design. Afternoon naps in a scanner do not perfectly simulate nighttime slumber, and while the sample size of 23 participants was adequate for these elaborate measurements, it remains relatively small. Future research will require larger, more varied cohorts and longer recording periods that encompass multiple complete sleep cycles, including deeper stages of NREM sleep. The team also aims to enhance their metabolic assessments and develop improved techniques to differentiate between the various stages of sleep.
Nonetheless, the research provides a rare insight into the ongoing activities of the sleeping brain, portraying it as significantly more active and intentional than the common perception of sleep as mere shutdown. The brain during NREM sleep resembles less a computer in hibernation and more a house at night: lights dimmed in most rooms, yet critical systems remain operational, poised to respond if necessary.
Nature Communications: 10.1038/s41467-025-64414-x
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