My dad worked the 11pm - 7am shift at a care facility a few years ago. He worked that shift Tuesday through Friday. The problem was that he tried to sleep on a "normal" schedule the other days. He was a complete zombie for the year he did that.
I kept telling him he had to sleep at the same time every day whether he worked or not or his body would never get used to it but he wouldn't listen. I'm only his kid; what would I know?
I was surprised it didn't kill him as he's in his 60's and has heart problems.