This may seem like a no-brainer, but it's not just the (sometimes) strenuous physical activity that helps you sleep so soundly. Post sex a chemical called oxytocin is released, which helps promote a restful sleep, according to Dr. Laura Berman. Next time she shakes her fist at you for passing out rather than cuddling, tell her it's not lack of love, but science that inspires you to snooze.