It’s also possible that sleepiness is just a “side effect” associated with a more evolutionarily important reason for the release of oxytocin and vasopressin. In addition to being associated with sleep, both chemicals are also intimately involved in what is called “pair bonding,” the social attachment human mates commonly share. The release of these brain chemicals during orgasm heightens feelings of bonding and trust between sexual partners, which may partially explain the link between sex and emotional attachment. This bond is favorable should the couple have a baby, as cooperative child rearing maximizes the young one’s chances for survival.
If you're having sex regularly (or even, um, "taking matters into your own hands," if ya know what I mean) and achieving orgasm, then it stands to reason that your stress levels might be a little lower. (You're a little more relaxed b/c you've been gettin' some...even if it's by yourself...you still get some of those feel-good chemicals.) If your stress levels are a little lower, you're likely preventing what they call the pregnenolone steal -- where so much of your pregnenolone goes toward making cortisol to keep up with all the stress and it gets siphoned away from making the other things pregnenolone ultimately leads to -- estrogen, progesterone, and testosterone.