Scientists say they have found the answer.
Now scientists think they know what's going on in our brains when we feel stressed. By studying the different regions of the brain that become active at different times, they have discovered that stress suppresses the part of the brain that makes us rational, according to Dr Amy Arnsten from Yale University School of Medicine in the United States. Very often in our lives we're faced with stresses, such as hearing bad news from our boss. Or it could be something as small as trying to drive to work in a hurry whilst thinking about an important meeting that morning and at the same time, trying to remember to stop off at the dry cleaners to pick up your suit to wear to a wedding that weekend. You hurry away, and even though you could concentrate very well before, you now find it's very difficult.