My initial thought is to talk to your doctor.
It could be some condition that resembles dehydration or that makes you get dehydrated specifically while you sleep.
I'd say try drinking more water before you got to bed, but then you'll probably have to wake up and pee in the middle of the night.
I'm 34 myself and noticing changes as I get older. Mostly in the department of my joints going bad from a lifetime of playing sports (not even from lifting, more from swimming, soccer, and basketball). I intend to continue with all the activity as long as I can, but now feel like I have to seriously address joint preservation while engaging in said activities.
Perhaps it's a sign that you're not getting enough water during the course of the day? Like, as you get older maybe you need to up your water intake?
My only other thought is that maybe there is something going on that is affecting your ability to retain water?
All just speculation, so take it for what it's worth. I'm sure you've already thought of most or even all this stuff. But since noone else replied, yet. I figured why the hell not chime in, right?
Good luck with the situation and keep us updated, I'm curious.