Pre-exam stress makes it more difficult for candidates to solve complex problems, a US report suggests.
Preparing for exams can be highly stressful for students
Researchers at Ohio State University looked at 19 first year students' performances two days before and a week after a classroom test.
They found the ability to deal with complex, open-ended questions improved markedly as stress subsided.
However, the students did better at simpler tasks, such as memorising numbers, while under more pressure.
During acute stress the body releases a compound called norepinephrine. Also known as "fight or flight" compound, it allows people to react quickly to an immediate threat.
Previous studies have shown it improves some types of mental activity, such as short-term memory.
But David Beversdorf, co-author of the report, said: "Even though norepinephrine may help a student recall memorised facts, it could hinder his ability to think flexibly."
For the short-term memory test, students were asked to write down a sequence of nine numbers they had just heard.
Another simple test involved thinking of a single word that linked to three others. For example, given "way", "ground" and "weather", they had to add the prefix "fair" to create "fairway", "fairground" and "fair-weather".
For the more complicated problem-solving test, students were shown a grid which contained a series of shapes and symbols.
They were asked to choose a shape or symbol which fitted in best with the others.
The overall performance here was far better after the classroom exam than before it.
Prof Beversdorf said: "There was a clear relationship between cognitive function and stress levels.
"The students don't think flexibly right before their exam, typically a time of great stress."
The findings are being presented at the annual Society for Neuroscience conference in San Diego, California.