I've had a reckoning with my caffeine metabolism recently. I fall asleep very easily, like head hits pillow and I'm zonked within a minute. But I often wake up at 3 or 4am and am unable to go back asleep.

What I've learned is that caffeine metabolism goes down with age and sleep gets lighter with age. Even if you can fall asleep easily, the residual caffeine in the middle of the night is enough to wake you out of light sleep. I made a tool to convince myself to cut back: http://jitterdone.com