You intentionally can't remember the last time you had sex but you definitely remember that it used to make you pretty sleepy when done correctly to exhaustion. Hell maybe without a partner it'll be easier to fully satisfy yourself, even. You decide to try masturbation.
It takes a while to figure out how it works, since this is something someone else should do, but the bio-feedback and growing 'developmental carrot' of the act make you a quick study. The first orgasm is pretty intense, but you're not sleepy. In fact, you feel like you've absorbed a lot more energy. Maybe you did it wrong.