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 it’s 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 have 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.
Update: I have a new favorite sleep mask that doesn’t have a traditional “strap” and instead just wraps around my head, covering both my eyes and ears. I actually got it as swag at a crypto conference 😅 so I can’t be 100% sure, but I think it’s this one:
It’s very soft (modal fabric but also soft batting inside), and it has extra padding around the nose to block more light. There is some kind of Velcro, but my hair doesn’t catch on it. The only drawback is that the fabric absorbs oils, which makes it look a little sketch, so I’m going to test out this black one (which I think is the same?)
It’s also worth trying to make your sleeping space light-proof. See Blackout blinds for more ideas (I even have a set I can travel with).
But even though I’ve got great light-blocking at home, I’m away a lot and would be lost without my eye mask in environments where I have less control.