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Researchers in France say mental fatigue is a good sign you need to take a break and rest.
A study has found a long day of thinking can feel as tired as physical work.
Researchers say long periods of taxing mental activity can cause a build up of the toxic by-product glutamate in the brain.
After 6 hours concentrating on computer-based tasks, participants showed higher levels of the neurotransmitter in their pre-frontal cortex.
Researchers in France say mental fatigue is a good sign you need to take a break and rest.