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The brain is a small fraction of body weight, yet it uses around 20 watts. That is like a light bulb powering constant thought and signaling.
Feeling tired at the end of the day is not only about muscles. Inside your head, billions of neurons keep messaging and burning energy.
The brain uses glucose and oxygen intensely because electrical signaling and chemical transmission are costly. Even at rest, background networks keep running.
This energy budget becomes more critical during attention and decision making. Small efficiency shifts can influence mental fatigue.
It is surprising what a light bulb worth of power can do. It also explains why sleep, food, and breaks matter so much.