Why does thinking make us tired? Researchers looked into this. When we think a lot, our brain releases a chemical. It tells us that we must rest. People who spend a long time thinking will feel tired. Resting helps the brain to return to normal. A 20-minute power nap can make us feel totally refreshed.

The researchers tested 40 people. Twenty-four of these had difficult tasks looking at a computer screen. The other 16 people did a similar, but easier task. Everyone worked for six hours. Brain scans showed that the group with the difficult task had more of the chemical in their brain.