I normally drink very little caffeine. I noticed in my freshman year of college that I was developing a tolerance, so I cut way back so that it would give me a kick when I needed it. In recent years, though, I've gotten very sensitive; I remember that somewhere in my LJ I have an entry about staying up until 4am or so with a single can of soda at GURPS. In addition, I have a job for which sleeping late is not a big issue. And I find that being naturally well-rested yields better productivity and clarity of thought than artificial substitutes; sleep deprivation plus caffeine tends to leave me wired and feeling as if my body was a garment from which the coat hangers had not been removed. In addition, Pittsburgh has really tasty tap water, and my tendency to penury makes that a beverage of choice, particularly in restaurants.
But I was really dragging on Sunday, and I decided to try compensating with caffeine. But I knew that I'd have to be careful to avoid coat-hangered wiredness and difficulty sleeping. So I took care to sip the Dr Pepper slowly, and only enough until I got the lift I needed.
It worked; I managed to fall asleep pretty readily, although not with the ease I'd expect from how exhausted I felt.
My dosage for the day to produce this effect: about a third of a 21-ounce bottle of Dr Pepper, spread over the course of about four hours of work. Apparently I really don't have a tolerance anymore.