Coffee gives you a lift because it buzzes your adrenal system. Now our adrenal system can't differentiate between a cup of coffee and a herd of buffalo bearing down on you, so it goes into the 'fight or flight' state. Your adrenal glands release adrenaline, cortisol and other hormones to keep you active, awake, alert, responsive. Brilliant! I had a bad nights sleep/big night out/a big meeting to prep for, so why not? Well if you keep taking out of your bank of energy, without ultimately giving it time to recharge you are going to burn out, physically and mentally. Common sense, rather than rocket science.
Research is still being conducted, of course, but studies have shown that ultimately that caffeine has a real effect on the reproductive system, affecting sperm motility and quality, delaying pregnancy rates (more than one cup a day halves chances of conceiving per cycle), it can be linked to miscarriage rates, exacerbates endometriosis and excessive consumption is going to weaken the immune system. In 'fight or flight' the body sends energy to the muscles rather the gastrointestinal tract, slowing it down, which can lead to indigestion. Alright, that's not so serious, but think about it if you don't go anywhere without a box of Rennies.
Quite simply, like any stimulant, you can use or abuse so if you are feeling depleted, keep getting colds and bugs, or you are trying to conceive then cut it out altogether if you can. It's not forever! At the very least cut it down to no more than one cup a day and give your system a chance to reboot.
Oh and a note on decaff. This is made by basically washing the coffee beans with formaldehyde to absorb the caffeine. Yum.