Simple: Drink Water.
Medicine: A recent study measured the cognitive abilities of 25 women after inducing mild dehydration. Researchers found that cognitive tasks were perceived as more difficult even though there was no significant reduction in the ability of the women to think. Dehydration caused headaches, fatigue, and difficulty in concentrating.
The moral of the story: Drink water! Relying only on thirst as a cue to drink water may not be enough, by the time you feel the urge to drink, dehydration may already be affecting how your mind and body perform.
“Mild Dehydration Affects Mood in Healthy Young Women,” The Journal of Nutrition