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 mental1 effort makes us tired.They found that when people think a lot,the brain releases a chemical called glutamate2.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 ...