Shut one eye to keep your night vision

Shut one eye to keep your night vision.

Apparently, this is a well-known thing in the military, but no one told me until we learned about it in the film Beneath Hill 60.

It's also a potential reason why pirates are known to wear eye patches—from popping up and down between decks—though it's rather hard to be sure. But I think the primary use case is going for a pee in the night.