From a BBC News article on the dangers of drinking too much coffee:
The Department of Health (DoH) advises people not to drink more than five single espressos - roughly seven instant coffees - a day, although individuals vary in their sensitivity.
Drinking one cup this morning was enough to make me wake up at 9 PM tonight sprawled sideways on my bed. Both my lights and my computer were on. The last thing I remember before waking up was feeling vaguely sleepy as I sat at my desk...
My general attitude about coffee is that it is generally useful, but can be dangerous (the migraines I got that spring!).