Scientists at Tohoku University have identified a previously unknown paradox occurring during rapid eye movement (REM) sleep, the stage most closely associated with dreaming. While the body rests during sleep, the brain remains highly active, particularly during REM sleep—a state already recognized as paradoxical because brain activity resembles wakefulness despite physical stillness. The new research reveals an additional layer to this paradox: blood supply to the brain increases during REM sleep, yet the levels of ATP, the energy molecule that directly powers neurons, actually decrease.
To conduct their investigation, researchers used a novel technique involving UV-curable resin to maintain transparent skulls in mice, enabling real-time observation of brain activity during natural sleep. Using wide-field fluorescence imaging, the team tracked changes in brain blood volume as an indicator of energy supply while simultaneously measuring neuronal ATP levels and astrocytic pyruvate, a compound linking glucose metabolism to brain energy processes. During non-REM sleep, the researchers observed that theta-band neuronal fluctuations could predict shifts in blood volume several seconds later, suggesting the brain actively adjusts blood vessel function in response to metabolic demands.
A distinct pattern emerged as the brain transitioned into REM sleep. Approximately 50 seconds before REM sleep officially began, brain blood volume started increasing, beginning in the posterior cortex and moving forward in what appears to be a large-scale preparatory process. Once REM sleep commenced, astrocytic pyruvate rose alongside increased blood volume, indicating greater metabolic fuel availability. Paradoxically, neuronal ATP declined during this same period despite the increased energy supply.
The researchers proposed several mechanisms to explain this counterintuitive energy pattern. Neurons may consume substantial ATP during REM sleep to support memory-related synaptic reorganization or communication between different brain regions. Alternatively, the transfer of metabolic resources between astrocytes and neurons may shift during this stage, or mitochondrial ATP production patterns may change. These findings suggest the dreaming brain operates under unusually high energy demands even as its fuel supply increases.
The research underscores broader principles about how animal brains manage energy differently from conventional computers, redirecting resources based on behavioral state and cognitive demands rather than distributing energy uniformly. Understanding these mechanisms may illuminate why sleep remains essential for memory consolidation and cognitive function.
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