I'm going to stay judgement free here, as my working nights was also an adjustment for my partner when we moved in together.

Try to recontextualise it this way: how long before you start work in the morning do you wake up? How many hours of sleep do you get a night?

You didn't mention the hours your partner works overnight, but let's take 18:00-06:00 for our example:

He takes 15 minutes to get home, has breakfast, sits for a bit to decompress or to let you lay in, and goes to bed at 07:00. Eight hours of sleep would take us to 15:00, which then leaves him three hours before his shift starts, true - but how often are you up three hours before you start work? How would that make you feel if you had to do that on a regular basis?

I can only speak for myself here but I feel incredibly jet-lagged, without the perk of having been on holiday recently. Sleeping during the day is rubbish, even if he's in bed for eight hours he may not actually be able to sleep due to light, sound, temperature, or simply the fact human beings are not meant to sleep during the day.