Some people seem to cope with stress without too much trouble, but for the rest of us, it can be a problem, causing us to perform below our best.

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.

Stress can cause the brain to wander

"Suddenly, you come across a bad car accident attended by an ambulance. You hurry away, and even though you could concentrate very well before, you now find it's very difficult. Your heart's pounding a little, you step on the brakes faster than usual and you're feeling distracted.

"You can't think about the morning's meeting and you forget to go to the cleaners. At work, you find yourself remembering the accident scene rather than your original plans for the day."

When we're stressed, we produce a family of nerve chemicals that act on our body, preparing our heart and muscles to respond to danger. But they also act on our brain, where they stimulate a region long known to be the seat of emotion.

This is why images of the accident scene keep flashing through our minds. They also stimulate the region responsible for repetitive and stereotyped actions, which is why our foot reaches for the brake pedal faster than normal.

Meanwhile, the same chemicals turn-off the part of our brain that allows us to cut out distractions and think effectively, which is why we can't remember the details we wanted to discuss at the morning meeting.

It also explains why athletes or footballers react badly after making a mistake - they can't forget their error to concentrate on their next move.

Dr Arnsten believes these responses to stress developed very early in our history to cope with the problems we faced then, such as being confronted by wild animals or natural disasters. But they're not appropriate for the problems we face in today's hectic world.

"We need to organise and concentrate and remember things better than ever now, in order to deal with the great complexities of modern life. So having this mechanism may be more troublesome today than it's ever been in the past."

Dr Arnsten's research may explain why children suffer behaviour and concentration problems when going through a stressful period in their lives, such as their parents' divorce.

It may also explain why people suffering from mental illness find that their condition worsens then they're stressed.

But Dr Arnsten feels that the most important message that comes from her research is that we can't help feeling stressed from time to time, and when we do, we should be kind to ourselves.