Have you ever wondered why thinking too much makes you tired? A new study has found out some answers to this question. Researchers from the Paris Brain Institute looked at why mental effort makes us tired. They found that when people think a lot, the brain releases a chemical called glutamate. Too much glutamate upsets the brain's functions. It means the brain tells the body it is tired and that it must rest. 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 allows the brain to return to normal. This explains why a power nap is good for us. A power nap can be as short as 20 minutes, but can make us feel totally refreshed.

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