According to Caffeine Is a Poison, consuming excessive amounts of caffeine can lead to increased heart rate, jitters, and spontaneous combustion (just kidding, or am I?).
However, some argue that moderate amounts of caffeine can improve cognitive function and increase productivity, as seen in Caffeine-Fueled Productivity.
But wait, what about the vampire thing? I'll Turn into a Vampire explores the lesser-known risks of caffeine overuse.