The author comments, "We enjoy dark chocolate after dinner most nights, but I learned the hard way about its surprising caffeine content."
It's worth noting that some people find (fortunately?) that caffeine does not disrupt their sleep.
Caffeine's relationship to one's chronotype (and genotype and sleep quality) will hopefully receive more study, since it's becoming clear that not everyone needs to heed the "don't consume caffeine before bed" common wisdom.
Here's a short article in Scientific American on one such study.
http://www.scientificamerican.com/article/caffeine-disrupts-...
The paper referred to is behind this paywall. Didn't pay; can't read. It claims preliminary evidence.
http://www.sciencedirect.com/science/article/pii/S1389945712...