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Forget expensive private tutors and brain-boosting vitamins. The key to exam success could be as simple as a glass of water.

Students who took a drink into the exam hall did up to 10 per cent better than those who did not – the difference between a grade.

Although it is unclear why a drink should help, one theory is that information flows more freely between brain cells when they are well hydrated.

Researchers said that drinking water may also calm nerves, while those who became thirsty during test time could be more easily distracted.

The study, which looked at hundreds of university students, compared whether they took a drink – such as water, coffee or cola – into the exam with their final marks.

The students' overall academic ability was then factored in, to ensure that the results were not skewed by the possibility that smarter students are also more thirsty.

Those who arrived armed with drinks did around 5 per cent better on average. But the improvement was even more marked among those just starting out at university, whose results improved by as much as 10 per cent – the difference between being awarded a first-class degree and a 2.1, the annual conference of the British Psychological Society's heard.

The type of drink did not change the results, meaning the students' performance could not be put down to caffeine or sugar.

Researcher Chris Pawson, from the University of East London, said: 'The results imply that the simple act of bringing water into an exam was linked to an improvement in the students' grades.' Dr Mark Gardner, of Westminster University, added that it was not clear why the greatest improvement was seen in new students.

However, it could be they were the most anxious, or having newly left home were more prone to wild nights out and so in greater need of hydration.

Earlier research from the University of East London has shown that children aged between seven and nine who drank water did better on tests of visual attention and memory.