Background: Evidence suggests that an insufficient fluid intake impairs cognitive performance. Drinking policies at schools--especially drinking during lessons--is a point of controversy. To provide a scientific base for this debate, more empirical evidence is needed on which aspects of fluid intake are crucial for cognitive performance. This study makes a contribution by investigating associations between quantitative and temporal aspects of fluid intake and cognitive performance in everyday school life. Methods: The study comprised 125 children (age: mean = 10.98 years, SD = 0.38). Amount of fluid intake and time span between fluid intake and completion of cognitive tests were determined on basis of self-reports. Cognitive performance was assessed by standardized tests. Results: Quantitative and temporal aspects were associated with cognitive performance: The more fluid the children consumed and the shorter the time span between their last fluid intake and test completion, the better they performed. Conclusions: The amount of fluid intake should be adequate and moreover the time span between intake and cognitive efforts should be as short as possible. Schoolchildren are thus recommended to drink at regular intervals and also during lessons.