I am no expert as our is our first dog!! However, our puppy is fast asleep in his crate while I work in the office this morning, I also popped out, he's been there two hours now so I will go into the kitchen soon tbh, not that he knows!
However, don't make a fuss at all when you go back. Ignore if possible, put the kettle on, put down the shopping etc. Only acknowledge when puppy is quiet. You are encouraging calm behaviour in the crate, it's no big deal etc.
We feed our puppy in there and cover with blankets at night (although he prefer to see us during the day but goes in to sleep and this might help him with day/night differential anyway?). Lots of toys in there, chews etc.
Initially, I threw loads of treats in, clicked, treats clicked, treats, closed door, opened again, clicked. Moved a step away with door closed, treated if calm, step further etc, went out of door, came straight back in, out/in etc. Treats, clicked, treats clicked etc.
It also helped us to move any other forms of bedding in the room (ie a piece of vet bed) so he had only a nice cosy place in the crate.
It did help I think that we were all around for Christmas and then, when he did need to be left, he was very secure in his environment and knew we were always popping in and out and he didn't need to worry. Although, that could have gone the other way I guess and I did worry about this so did the above.
I don't close the door unless I need to leave him though, he's free to go in and out as he pleases.
Experts will be along soon!