Have you ever thought why thinking makes us tired? A new study has found some answers. Researchers from the Paris Brain Institute looked at why mental effort makes us tired. When people think a lot, their brain releases a chemical called glutamate. Too much of this changes the brain's functions. It tells the body it is tired and that we must rest. People who spend six hours working on a task that requires thinking are more likely to feel tired. Resting helps the brain to return to normal. This explains why a power nap is good for us. A 20-minute power nap can make us feel totally refreshed.

The researchers looked at the behaviour of 40 participants. Twenty-four of these performed challenging tasks. These included looking at a computer screen and matching different letters. The other 16 people did a similar, but easier task. Both teams worked for six hours, and had two breaks. The researchers scanned the brains of the participants. The group with 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."