The brain uses an extraordinary amount of energy compared with most other organs. Even so, it can continue processing information when energy is limited by adjusting how available resources are used. Scientists are still trying to understand how the brain manages this energy budget as it moves through different internal states.
Sleep offers a useful way to study that question. Although the body is resting, the brain remains active, especially during rapid eye movement (REM) sleep, which is closely associated with dreaming and memory processing. REM sleep is often described as "paradoxical sleep" because the body is mostly still while brain activity resembles wakefulness.
Researchers at Tohoku University have now identified another REM sleep paradox. During this stage, the apparent energy supply to the dreaming brain increases, yet levels of the energy molecule neurons directly use actually decline. The findings were published in Communications Biology.
"Ever felt exhausted after a vivid dream?" asks Professor Ko Matsui of Tohoku University. "Sleep may appear peaceful, but the brain is highly active -- especially when dreaming. We were intrigued by this paradox, and wanted to look into the scientific basis behind why dreaming is somehow tiring." Watching the Sleeping Brain in Real Time To investigate what happens to brain energy during sleep, the researchers used a UV-curable resin to keep the skulls of mice transparent, allowing them to observe the brain during natural sleep.
Using wide-field fluorescence imaging, they tracked changes in brain blood volume as a sign of incoming "fuel" supply. They also measured neuronal ATP, the energy molecule that powers neurons, along with astrocytic pyruvate, a key compound that connects glucose from the blood with energy metabolism in the brain. Non-REM sleep is well known for strong neuronal activity in the delta-band frequency, but smaller theta-band fluctuations also occur.
The researchers found that these theta-band changes could predict shifts in brain blood volume several seconds later. That pattern suggests the sleeping brain adjusts its blood vessels in response to changing neuronal activity and metabolic demand. The Brain Prepares for REM Sleep in Advance A different pattern appeared as the brain moved from non-REM sleep into REM sleep.
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