First, caffeine is a vasoconstictor (it shrinks your blood vessels) thereby, it increases your heart rate and blood pressure. This is why caffeine is so effective for treating headaches.
Second, caffeine is also a diuretic (it makes you pee).
Therefore, if you are already dehydrated, then the caffeine will not have enough blood volume to shrink down the vessels to increase your blood pressure, and the caffeine will increase your dehydration to make you more tired.
Drink at least 3 to 4- 8 ounce glasses of water before drinking one cup of coffee. You will run to the bathroom in 20 minutes, but you see that you will have more pep and energy and use the 3 to 4 rule with every cup. You will find that you need less coffee to give you more energy