I had an extremely stressful year at work, many years ago, and realized one Saturday that as a result my caffeine intake was way too high. I quit cold turkey, and oh the headache. It felt like someone had hit the back of my head with a mallet. I do drink coffee now, but at noon I switch to decaf (same for tea). I almost never drink soft drinks, so coffee/tea is my caffeine source.
For the tea drinkers - when I took organic chemistry we extracted caffeine (nice white crystals). We used tea, not coffee. There is definitely lots of caffeine in tea ;-)