Get one, use one. It's wild to me how much light gets through my eyelids. I think it probably makes sense to try to get as much of your sleep as you can while the sun is down (in part to enjoy the sun while it's up!), but in most parts of the world, you're unlikely to experience truly dark night skies (though worth venturing out to look at the stars when you can :)
Tip: put your mask on before going to sleep, rather that letting the sun wake you up and alert you to the fact that you need your mask.
I also know people who prefer masks that give pockets (such that you can open your eyes without obstruction), but I have such a small nose bridge that the light comes in along there unless I manage to lie very very still, so mine only has extra padding below my eyes. But maybe worth looking into.