Caffeine Absorption Caffeine Capsules - 200mg - 84-120 minutes
So maybe you took the caffeine, were already tired so you napped(2 hours or 120 minutes) just as it was kicking in full blown and then woke up a little later with the effects already in action, but not sleeping the full amount.
Since the capsules take longer to kick in than liquids and gums, you fell asleep before it fully woke you up. And since some say that caffeine works less if you are caffeine tolerant then this may explain.
According to Mens Fitness it could be a variation in your genes (CYP1A2 genes) that show how you metabolize caffeine. There is a test for that apparently, but I don't know how accurate that is.
The 7 gene test consists of a panel of seven genetic markers that
enable your healthcare professional to provide you with personalized
nutritional recommendations based on your DNA. This test determines
how your body responds to vitamin C, folate, whole grains, omega-3
fats, saturated fat, sodium and caffeine.
But then again, there appears a lot of reasons to why caffeine makes some people sleep:
For most young, healthy adults, caffeine doesn't appear to noticeably
affect blood sugar (glucose) levels, and consumption up to 400
milligrams a day appears to be safe.
- Some also say it has to do with vasoconstriction and dehydration due to diuretic effects of coffee.
The above other reasons don't seem to be the case in your case, but they are interesting possibilities.