A study of university students found that those who brought drinks, especially water, with them as they sat their exams performed up to 10 per cent better than those who did not.
Psychologists said it was unclear why drinking water would improve your performance but said that being better hydrated could have a helpful impact on the brain, and knowing you had a bottle with you might make you feel more reassured.
The researchers studied hundreds of university students in their first and second years of degree courses and at pre-degree "foundation" level and observed what drinks, if any, they brought into exam halls with them.
Their study, presented at the British Psychological Society annual conference in London on Wednesday, found that those who brought drinks in with them averaged five per cent better in exams.
The students' coursework and predicted grades were considered in the calculations to ensure it was not simply the case that more intelligent students were more likely to bring drinks with them.
For students at foundation level, grades improved by 10 per cent – the difference between a 2:1 and a first class degree – while there was no significant benefit for those at second year level.
Mark Gardner of Westminster University, one of the collaborating researchers, said: "If it turns out to be that the effect is indeed larger for younger students we think it could be important – those students could be more anxious and the water is having an effect on exam anxiety, perhaps through beliefs about the positive effects of water.
"Or it could be that the younger students might not eat as healthily, have possibly been out late the night before, and might need to drink more water to stop them being dehydrated."