Up until about a year ago I drank coffee whenever I wanted, all day and every day. Then my doctor said I should cut down to two cups. So I've dutifully cut down, although I still generally have three cups or so each day. Before yesterday that hadn't been an issue.
Yet sitting at my computer going through e-mails and such and such, I went through four cups in an hour on an empty stomach. It didn't bother me at first, but by the time I got to my Shop I was nervous and jittery and talking a mile a minute. I know, that last symptom will draw a laugh, but I could actually hear that I was prattling on by then.
So it seems that my caffeine tolerance has gone way down. Stupid doctors.