SIGNS OF CHRONIC DEHYDRATION
If it's 15 to 20 beats per minute higher than usual, your heart is compensating for reduced blood volume by beating faster.
If you get dizzy standing up, blood isn't reaching your brain fast enough. Low blood volume is to blame.
Pinch the skin on the back of your hand for a few seconds. If you're hydrated, it'll snap back immediately. If you're not, it'll be slower.