It's the caffeine and its side effects. It can cause sleep disorders, anxiety, and so on. Of course, not everyone encounters these problems with their coffee-drinking. See the Wikipedia entry for caffeine if you want more details though.
yes, mostly sleep disorder, the intense crash and burn of coffee compared to a more mellow effect of for example green tea, and the tummy problems.
especially the crash/burn + sleep problems though, which lead to anxiety, which leads to either depression, or in my case a kind of jittery angst-fueled hyperactivity.
