Always hard
I do remember having this issue back when I first started sleeping naked. Are you new to nudism? It might just take some time. I think I'd also be different if I slept alone and didn't have sex every night. Relieving that pressure is a good way to pass out whether it be with someone else or solo - might help you with the mess when you wake up as well!
As has been mentioned you may just need some relief. Have you tried masturbating before bed? I occasionally get a lot of pent up energy too so I'll do that when I need to calm down and it works really well. I take it you're always naked at home like I am? When you're naked it's especially relaxing. Aside form countless other reasons I love living naked, I just really enjoy being as naked as whoever I'm viewing as my "subject matter" when "relieving tension," lol. Just find whatever sexy material appeals to you and have fun!
NOTE: I do not at all associate being a nudist with sex. I am simply recommending something that I would still recommend had I not been a nudist. We are sexual beings just the same as clothed people, and like with clothed people we know there is a time and place for everything. We just don't have to get undressed for it, lol!