Have you ever thought why thinking makes us tired? Researchers from Paris looked at why mental effort makes us tired. When we think a lot, our brain releases a chemical. Too much of this changes the brain. It tells the body that we must rest. People who spend a long time on a task are more likely to feel tired. Resting helps the brain to return to normal. 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 people. Twenty-four of these had challenging tasks. These included looking at a computer screen. The other 16 people did a similar, but easier task. Both teams worked for six hours. The researchers scanned the participants' brains. The group with the more difficult task had higher levels of the chemical in their brain. The study's author wants to find out about how the chemical levels are restored.