One of the first things to be affected by dehydration is our brains - which is why we get headaches (often the first sign we have that we need a glass of water) and various other symptoms to the point where we find it hard to think, hard to respond, and hard to be rational...

Def, do as the others have already said - stay hydrated, take more water than you think you'll need and sip at it often rather than huge swigs occasionally.