Students should resist taking sips of cold water to calm their nerves during exams, according to researchers at Bristol University. A recent study has shown that simply drinking water at the wrong time can impair mental performance.

The study in Appetite (2001;36:57-8) asked 60 volunteers to rate how thirsty they felt. Their reactions were then tested with a task that involved pressing buttons in response to prompts on a screen. The participants either drank nothing before the test or had a cupful (330 mls) of tap water, chilled to 10°C. Those who drank the water recorded scores 15% lower than those who drank nothing.

Peter Rogers, the experimental psychologist who led the study, said, "Many students take a drink into a long exam. If you are not thirsty and use the water as a distraction or comfort tool it may have a detrimental effect on your performance."

The researchers also found that drinking when you are genuinely dehydrated may improve performance. The volunteers who were thirsty at the beginning of the test and took a drink performed 10% better than those who drank nothing.

Peter Rogers said, " If you only have a drink when you are feeling hot and thirsty in an exam then it is probably needed and you will perform better for it." He thinks that the temperature of the drink may partly explain the effect. "When drinking water that is very cold the body has to divert resources to deal with the local cooling effect in the gut."

The finding that it is detrimental to drink water when not thirsty can also be applied to practical procedures, especially those where fast reactions are needed, such as driving a car or performing surgery. Mr Rogers said, "It is difficult to assess what effect it would have in these situations, as when under great pressure we draw on other compensatory resources to help our performance." He concludes, " The biggest lesson from the study is to listen to your thirst."