We think of stress a bad thing. While this is usually true, there is evidence that moderate amounts of stress can be a good thing. There is an "inverted U" shaped curve between anxiety and performance. When we are totally relaxed we do not perform at our best on most tasks. A little anxiety and stress can motivate us and help us perform better. Too much stress impairs performance, however.
Test anxiety is a good example of this. Most of us have had the experience of walking into an exam after studying, and feeling prepared. Several questions into the test, when we encounter material which we don't remember, our anxiety level increases. We may find that we then forget things that we studied.
If we had taken the test while completely relaxed we also might have scored poorly. Being moderately alert and motivated - along with the accompanying moderate autonomic nervous system elevation - results in the best performance on many tasks.
Memory problems have also been found in mice who are stressed. Mice who have been trained to run a maze often forget how to run the same maze when under stress. Researchers also found elevated levels of a hormone similar to the human hormone cortisol - sometimes called the "stress hormone." Elevated cortisol levels may be one reason that too much stress impairs performance on some tasks.
Hans Seyle is a pioneer in stress research. He calls positive stress "eustress" and negative stress "distress." Most of us enjoy a challenge, but we don't enjoy too many of them at once. Too much eustress can easily become distress. Moderation and balance seem to be important for our minds and bodies to cope with the demands of life.