The digging is normal behavior. Do you have them on sand?
Dragons usually burrow for the night when they're in the wild, or they'll find a nice little cave. Him digging in the corner is completely normal, however if you have them on sand, I would highly reccomed changing it out for a non-particle substrate. Slate tile works awesome. Also, if that particular dragon likes to cuddle down for sleep, you could give him (or her I guess) a nice comfy blanket or towel that she can snuggle in. You could also make him a bed.
One of my dragons used to go her his bed at night and wiggle under his blanket with his head on the little pillow. Too cute.