Dreaming May Deplete Brain Energy Despite Increased Fuel Supply

Dreaming during rapid eye movement (REM) sleep may consume significant brain energy, even as the brain's fuel supply increases. Researchers at Tohoku University observed this paradox in mice. They found that while blood supply to the brain rises before and during REM sleep, the immediate energy available to neurons decreases.

The brain uses a large amount of energy. It adjusts how it uses resources when energy is limited. Sleep provides a way to study this energy management. The brain remains active during sleep, especially during REM sleep. This stage is linked to dreaming and memory processing. REM sleep is often called "paradoxical sleep" because the body is still, but brain activity resembles wakefulness.

Scientists used a transparent skull technique to observe the brains of sleeping mice. They tracked changes in brain blood volume, a sign of incoming fuel. They also measured neuronal adenosine triphosphate (ATP), the energy molecule for neurons. Additionally, they measured astrocytic pyruvate, a compound connecting blood glucose to brain energy metabolism.

Brain blood volume began to increase about 50 seconds before REM sleep started. This change moved from the posterior cortex forward. This suggests a large-scale process prepares the brain for REM sleep. During REM sleep, astrocytic pyruvate also increased. This indicates more metabolic fuel or increased glycolytic activity in astrocytes. However, neuronal ATP levels fell at the same time.

This drop in ATP may be due to high energy demands during REM sleep. Neurons might use large amounts of ATP for memory-related synaptic reorganization. They could also use it for communication between the hippocampus and cortex. Another possibility is a change in metabolic resource transfer from astrocytes to neurons. Mitochondrial ATP production might also shift during this stage.

These findings suggest the dreaming brain operates under unusually high energy demands. This occurs even when its fuel supply increases. Understanding this balance between energy supply and consumption could explain the efficiency of biological intelligence. The research adds to the understanding of why sleep is vital for both the body and the brain.