Caffeine

So, there might be more caffeine in your average shot of espresso coffee, but because it's so concentrated the body doesn't actually take much of it up. On the other hand, a coffee that's diluted with lots of milk delivers much more caffeine to your body, because it sticks to the fatty milky bits and doesn't just pass straight out of you again.

Which is why you won't have difficulty sleeping after an espresso after your dinner.