Hey guys; I'm wondering why we get tired after sex. I remember reading something in a physiology book about how when you are trying to reach orgasm (i.e., sex) either parasympathetic or sympathetic takes over, but when you have the orgasm the other one takes over. Is the taking over of one of those why we get tired? Is there any evolutionary reasons for this? Perhaps the sleeping allows you to devote energy into rebuilding your sperm collection, which ensures you of being ready to get more copies of your genes into the next generation. I know you would make the sperm regardless, but perhaps this helps? I guess I want to ask this too. How comes sometimes not having sex can keep you awake?