Large doses

The culprit is cortisol, one of a number of hormones produced by our bodies when we are subjected to severe stress for long periods. In large doses, the chemical reduces the body's ability to supply energy to the brain.

In a controlled experiment, researchers at the Washington University School of Medicine gave high doses of cortisol to a group of volunteers, while another group received a placebo.

Memory and learning

After four days the group which had taken the high dose of cortisol showed a significantly lower ability to memorise a written passage. This type of memory involves several brain regions, including the hippocampus, a sea-horse shaped brain structure related to memory and learning.

The cortisol levels produced in the study were significantly higher than those that occur during an average bad week. The amount of the chemical administered corresponded to extreme trauma, equivalent to abdominal surgery.

Chemical affects seat of memory

To suffer significant memory loss, a student would have to suffer a death in the family or some other catastrophe on top of 'exam nerves' to notice much difference in their ability to remember things.

"We saw memory impairment only in the individuals treated with the high dose and only after four days of exposure. The good news is it appears that it would take several days of stress like major surgery or severe psychological trauma in order to produce memory impairment." says John Newcomer, Assistant Professor of Psychiatry and Psychology.

More experiment needed

It is still not clear how much stress has to be present before memory suffers. Professor Newcomer believes there may be some effects from long-term exposure to slightly lower levels, though these experiments have not yet been done.