[jacobmuller]

Over use of caffeine can give you palpitations (irregular heartbeat). An increased heart rate can make you feel tired.
Coffee increases blood pressure - decaf does the same.
Coffee may be used to wake you up but an apple is actually more effective (don't know the Why but chewing, sugar, and no increased heart rate sounds good to me). Caffeine can actually cause fatigue - it works like a burst of adrenaline. Adrenaline isn't all nice - diarrhoea, vomiting, tremors...

A coffee use study showed that it improved work rates by <5%; coffee, no sugar, led to more work but more errors. Coffee plus sugar got more work done due to less errors.

Withdrawal will leave you miserable for a few days; possibly migraine level headaches. This about the only thing that's measurable in GURPS terms unless you want to count a strong dose of coffee/caffeine/taurine as temporarily negating 1FP but having to be paid back double...

PS taurine "energy" drinks should be diluted with an equal volume of water.