A team of neuroscientists at Tohoku University has identified an “energy paradox” in the brain during rapid eye movement (REM) sleep, revealing that neurons can experience an energy shortfall even as blood supply to the brain increases.
In a study published in Communications Biology, researchers found that neuronal levels of adenosine triphosphate (ATP), the main energy source used by cells, declined during REM sleep in mice despite increases in brain blood volume and astrocytic pyruvate, a metabolic fuel source.
The research team, including Professor Ko Matsui and lead author Yusuke Takahashi, simultaneously tracked brain blood volume, astrocytic pyruvate and neuronal ATP in live mice during natural sleep using fluorescence imaging through the intact skull.
The data showed that brain blood volume began increasing about 50 seconds before REM sleep was conventionally detected. Once REM sleep began, astrocytic pyruvate increased while neuronal ATP declined, revealing a mismatch between increased blood supply and the energy immediately available to neurons.
Researchers said the ATP decline could be associated with energy-intensive processes during REM sleep, including enhanced hippocampal-cortical communication and memory-related synaptic reorganization. However, the study did not establish what caused the ATP decrease, and researchers also considered changes in energy transfer and ATP production as possible explanations.
The findings suggest that increased blood flow does not necessarily mean neurons have more usable energy. Instead, the brain may use different energy-allocation strategies depending on its behavioral state and neural activity.
Matsui has said the possibility that REM-related neuronal energy depletion contributes to tiredness after vivid dreams remains speculative and will require further research.
