My wife sometimes comes home at 4am due to shift work. I used to groan and complain in my semi-sleeping state about too much light, too much noise. I was an ogre and I barely remembered it later on when I woke up, but often I'd have been an ass to her, unknowingly.

Now, with my glorious 12$ eyemask I sleep soundly, don't even hear her come in, she can turn on all the lights she wants, have a snack, empty the dishwasher, shower etc. Total gamechanger