MPT, Thanks for the suggestion. I think, unfortunately, his response would be "I don't know" which seems to be (sorry for the gross generalization ahead) a frequent reply of the LD person to any form of inquiry. For the record, I really DO believe that he doesn't know why.
Hairdog, I think that you are right. He really succumbs to tiredness easily. He is one of those people who fall asleep within seconds of his head hitting the pillow.
Plus, here is the thing: There is no burning NEED for him to fight that tiredness and stay awake. Imagine, if you will, that your W is ready for sex any time, any place. Now this might sound heavenly right now...but really and truly...it doesn't give one much motivation to stay awake for it! His attitude is "why does it matter, it will be there tomorrow night". He succumbs to the tiredness because he CAN. It depresses me to be somewhat 'dispensable' to him. Well that's not the right word but I hope you get what I'm saying. The only way I can change this dynamic is to NOT be available every single night and I don't see that happening without a drastic libido change, or some serious play acting on my part, both of which do not appeal to me.