Why do pirates often wear an eye patch?

It takes an average human eye about 25 minutes to fully adapt from bright sunlight to seeing in complete darkness—if a pirate was fighting on deck in the light, then had to go under the deck to fight downstairs where it is usually pretty dark, that would be a long time to go without being able to see. read more

Pirates wore eye patches so one of their eyes would constantly be used to the dark. That way if they were attacked at night, they could whip off the eye patch and not gave to waste crucial time stumbling around before their eyes adjusted. read more

Some assume pirates wore eye patches to cover a missing eye or an eye that was wounded in battle, but in fact, an eye patch was more likely to be used to condition the eye so the pirate could fight in the dark. read more

Pirates frequently had to move above and below decks, from daylight to near darkness, and Sheedy says the smart ones "wore a patch over one eye to keep it dark-adapted outside." When the pirate went below decks, he could switch the patch to the outdoor eye and see in the darkness easily (potentially to fight while boarding and plundering another vessel). read more