doesn't happen to me to be honest, I tend to feel energetic and full of energy when I'm done (although if I try to do anything I quickly find out that my body has a macho "BRING IT ON" ego problem) but then again I'm guessing I don't ride as long as most of you guys on here.
From what little nutritional knowledge I have I would guess it might be a post workout meal problem, maybe try having a small amount of fast acting carbs followed by some slow releasing carbs with your protein and replace your salts and water levels. I've heard that not doing a proper cool down can cause a "slump" so to speak so that might be something to look at. I'm sure some people on here will be able to give you a better option as far as nutrition goes.
Another option is to convince the wife to give you a massage
