Heat plays a role for sure. Golf does it to me more than biking, but I get weary and sometimes find my eyes closing in mid afternoon after hard exercise on a hot day. (Those who don't play golf often don't think if it as exercise, but walking the course on a hot day leaves me in about the same state as a 50 mile ride). For me though, it doesn't last and by the evening I'm no longer really tired and I still end up going to sleep after midnight.