I am a night person. I stay up until 4 and 5 am on a regular basis...
I have a hard time winding down once I come in from work late at night when I have worked that late shift. I tried to explain it like this. When you come home from work at 5 pm are you ready for bed? Do you go straight to bed and sleep until a designated time then set your alarm to get up? No, you have time to unwind, watch TV, read, get on the computer, take a shower, etc...
If he is going directly to bed when he gets off work then after he sleeps, UNDISTURBED, for a full 8 hours he should probably be getting enough rest.
We all know what it's like to be woken up every little bit when we have little ones so you know what I mean when I say uninterrupted sleep. If he is waking every little bit by kiddo or you, then he is not getting rested. He is getting choppy sleep and that is not rejuvenating.
I would also suggest he talk to the doc about a sleep study to see if he has sleep apnea. The Bi-pap or other device they sleep with really helps them get the oxygen they need while they are sleeping and it sure makes a difference in their sleep patterns.
So, my advice. If he needs the rest he needs the rest. If he truly is going to bed and sleeping soundly, consistantly, without interruption from 1am to 2 pm something is wrong. No one needs that much rest.
If he were winding down and going to bed around 4 or 5 like I would on his schedule then I would expect to sleep until the after noon every day.
His sleep pattern should be the same consistently. If he doesn't work his body is still used to the sleep pattern so even if he doesn't work that night he is still going to sleep his normal pattern.