There is evidence that mild dehydration has a negative effect on the brain's performance.
Caroline Edmonds and colleagues from the University of East London and the University of Westminster here report that drinking water can improve performance on tasks that require a rapid response, particularly when thirsty.
They tested 34 adults, who had not eaten or drunk anything overnight, for memory, attention, learning, and reaction time. Subjects were tested on two mornings: once after they had consumed a cereal bar and water, and once after eaten a cereal bar only. Reaction times were up to 14% shorter after drinking water, especially for those who felt thirsty.
Unexpectedly, performance on a complex-rule-learning task became slightly worse after drinking. Future research will have to determine why drinking water can be beneficial for some cognitive tasks, but not for others, say the authors.