I find I can still sleep alright 4 hours after taking a pill, like I would with a cup of coffee. I keep to that schedule of every 4-5 hours over the course of the day.

Maybe the coatings in your particular pill were different. I found gelatin capsules kicked in slower than pressed tablets that began to dissolve when they hit my saliva, but still lasted about the same. My tolerance to caffeine has always been high, though.