Is Hydration good for cognitive function?

The evidence is still emerging, so it's too early to say for certain. Hydration's evidence base for longevity is thinner and more mixed than commonly assumed, with the most direct test of the question returning a null result. That's reflected in an evidence score of 30/100 for its role in cognitive function, drawn from 4 studies, with an estimated impact on longevity rated limited (12/100).