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Why Does Thinking Hard Make You Tired?

After studying or taking a test, do you ever get tired? Why do we get sleepy after thinking hard?

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Have you ever felt extremely physically exhausted after a long day of doing mental tasks, like taking an exam? You're not crazy: there are some scientific explanations. For starters, our brain only makes up 2 percent of our body weight, but it uses up 20 percent of the energy we take in. A study published in Proceedings of the National Academy of Sciences USA found that two-thirds of the energy the brain uses goes to the firing of cells, and the other third is brain maintenance. Most of the energy powering the brain is in the form of glucose from the things we eat. When neurons fire, they absorb extra oxygen and glucose from nearby capillaries. Scientists, therefore, think that when performing a difficult mental task would burn more glucose. This would result in less glucose in the blood for everything else, hence that feeling of exhaustion after a long day of thinking.

Another recent study published in Human Factors: The Journal of the Human Factors and Ergonomics Society found that people get more exhausted when exerting both physical and mental faculties than when just simply exerting physical functions. Researchers discovered that when performing both physical and mental tasks, there was strong activity in the prefrontal cortex and there was lower blood oxygen levels after a combined physical and mental task than when just doing physical work. Following this line of thought, eating foods high in glucose should improve performance. A review published in the European Journal of Pharmacology found that while glucose can improve memory, it depends on the dose and the person.

Learn More:

The body and the brain: Impact of mental, physical exertion on fatigue development (Science Daily)

"Do you ever notice how stress and mental frustration can affect your physical abilities?"

Why Does the Brain Need So Much Power? (Scientific American)
"It is well established that the brain uses more energy than any other human organ, accounting for up to 20 percent of the body's total haul."