I drink coffee/espressos each day, probably too much. Just recently after not having my usual Americanos I had a massive headache before bed. Advil did nothing so I figured it must be a caffeine withdrawal headache. They're hell and there's no way I can sleep with one so I made a shot of espresso and downed it quickly. Within 30 mins my headache went from a jackhammer pounding my skull to a distant woodpecker, and after another 30 mins I was asleep and slept fine. The big confounding factor with caffeine studies might be tolerance.