Dehydration causes apathy. A glassful of water in the time you feel apathic and like better doing nothing could make wonders and surge of new energy.
What you can do to avoid dehydration:
- drink plenty of fluids, how much depends on climate and many other things
- avoid caffeinated beverages (tea, coffee, ...)
- avoid carbonated beverages
- avoid alcohol
- learn to notice when your body is dehydrated and needs more water (this ability is natural but often gets lost through time)
I don't have the numbers handy, but read it in numerous sources - dehydration is a rather large problem with our population today, most of the people affected not noticing they are "just" dehydrated in fact.