Bob worked all night: he'll sleep from 9 am to 6 pm. What expression can one employ to wish him a "good night" right before he goes to sleep?
When I was on a team that sometimes was called upon to work well into the night, coworkers often bid one another farewell with "Good work" (meaning, work is done) or "Rest well" (meaning, I'm not telling you must go to sleep now).
When we were feeling ill-used, folks might say a snarky: "Enjoy your time off" (as if being allowed to sleep after a long work shift were a vacation).
"I'll see you when I see you" was also heard, meaning the person should rest and relax, and not hurry back to work.
Direct references to sleep, bed, etc., are often avoided in business settings, as they are intimate subjects.
Sleep tight (The origin of this strange phrase is that in medieval times beds were wooden frames with ropes across them. To stop the ropes stretching too quickly, you would de-tension them when you woke up and re-tension them at bed time.)
Not Good rest or good sleep. Not in Britain anyway. That's very clunky.
As someone who does have an irregular sleep schedule due to sleep disorder, I simply use myself (and also have been wished) "good night" and "good morning" regardless of the time of day. After all, I'm not taking just a nap, I'm having my "full night's sleep", even if it's noon outside the window. And my morning is when I wake up from that sleep, I go do my morning routines and everything else associated with morning.
I also use time dependent words like "breakfast", "lunch" and "dinner" relative to when I woke up. So my first meal is breakfast, next lunch and then dinner. So I may be eating lunch at midnight for instance.
However, whenever I'm interacting with the general public, I use the normal greeting for that time of day.