Your brain is mostly made of water and is highly sensitive to dehydration. Studies have shown that even mild dehydration (1-3%) can have an adverse effect on memory and feelings of anxiety.
In another study, loss of fluids after exercising negatively affected the mood and concentration of the study participants, and increased the amount of headaches subjects experienced.
Yes, you read that correctly, dehydration can cause headaches and migraines (more below)!
A 1-3% loss of fluids in your body can happen with just normal everyday activities. So to keep your brain functioning at its best, make sure you drink plenty of water all day.
https://loseweightbyeating.com/benefits-of-drinking-water/