Feeling off? A cup of water, not headache pills, could be all you need.

Severe dehydration can do some pretty crazy things to your mental status, invoking extreme confusion and delirious thoughts. But what happens to your brain when you're just mildly dehydrated—a state that many of us are in every day? To figure this out, researchers conducted a small study of 25 young women, comparing groups that were hydrated to ones that were in need of water.

In the three-day study, women were dehydrated through three 40-minute treadmill workouts and in a separate treadmill workout after taking a diuretic. In the final test, they worked out on a treadmill but were allowed to properly hydrate during the exercise. During and after each bout of exercise, the women took a series of cognitive tests.

The researchers combined the results from the exercise-only and exercise-plus-diuretic sessions and compared those results to results from the adequate-hydration session. The average degree of dehydration was a 1.36% decrease in body mass. When the women were dehydrated, they reported less vigor, more fatigue, more mood disturbances, reduced ability to concentrate, increased perception of task difficulty, and greater severity of any headaches both when they were at rest and while exercising.

Interestingly, the women could not themselves distinguish between being dehydrated and adequately hydrated. While there's not a one-size-fits-all recommendation for how much water to drink daily because different factors like humidity, physical activity levels, altitude, and water content in food play a role, a 2007 Mayo Clinic report found that women who are adequately hydrated take in the equivalent of about 9 cups' worth of beverages a day.