If you feel a rush of lightheadedness when you stand up quickly after sitting down to stretch, it’s a good sign that your body’s low on H20. Dizziness is caused by a decrease in blood flow to the brain. And when there’s not enough water in your blood, blood volume and pressure both drop.
What about feeling run-down? Well, virtually every cell in the body needs water to function, so when you’re lacking liquid, your body has to work extra hard to carry about basic functions, hence the reason finishing that last mile or rep feels almost impossible.