Sleeping w/ a mask on isn't an issue, no different from wearing one for >1hr. Only thing to watch out for is bring spares in case you drool. Also, neck pillows can move the masks around depending on the size. Wasn't long haul but three 3-3.5hr flights in 1 day and it was fine.
I'm honestly amazed by how much people complain about the slight discomfort of wearing a mask. I've never seen so many grown men whine so much about something that isn't that bad lol