Hydration & Mental Performance

Why even small dips in water matter

Your brain is roughly 75% water. Some studies have associated mild dehydration, around 1–2% of body water lost, with dips in attention, working memory, and mood, though the magnitude of these effects appears to vary by task and person. By the time you feel thirsty, your body has already been losing water for a while.

Source

Ganio, M. S., Armstrong, L. E., Casa, D. J., McDermott, B. P., Lee, E. C., Yamamoto, L. M., Marzano, S., Lopez, R. M., Jimenez, L., Le Bellego, L., Chevillotte, E., & Lieberman, H. R. (2011). Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition, 106(10), 1535–1543.

In real life...

Don't wait until you're thirsty to drink. Thirst is a lagging indicator that your body is already losing water. Keep a water bottle visible at your desk as a cue. Hydration needs vary widely with body size, activity, and climate, so don't fixate on a specific cup count. Even drinking a glass of water before a mentally demanding task can give you a small performance edge.