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Have you ever why thinking too much makes you tired? A new study has out some answers to this question. Researchers from the Paris Brain Institute looked at why mental effort makes us tired. They that when people think a lot, the brain a chemical called glutamate. Too much glutamate the brain's functions. It means the brain tells the body it is tired and that it must . People who spend more than six hours working on a task that requires a lot of thought are more likely to feel tired. A period of rest the brain to return to normal. This why a power nap is good for us. A power nap can as short as 20 minutes, but can make us totally refreshed.

The research team at the behaviour of 40 participants in their research. Twenty-four of these had to challenging tasks. These looking at a computer screen and matching different letters that . The other 16 participants did a similar, but easier task. Both teams for six hours, and had two ten-minute breaks. The researchers the brains of the participants. They found that the group who had the more difficult task higher levels of glutamate in their brain. Study author Antonius Wiehler said: "It would great to out more about how glutamate levels are restored." He asked: "Is sleep helpful? How long do breaks to be to have a positive effect?"