From the Wall Street Journal:
Adjusting to the dim light next to your bed should only take seconds. But when going from bright light to maximum darkness, studies have shown, eye sensitivity continues to change for up to 25 minutes, he says.
Just ask a pirate.
"Ever wonder why a pirate wears patches? It's not because he was wounded in a sword fight," says Dr. Sheedy. Seamen must constantly move between the pitch black of below decks and the bright sunshine above.
Smart pirates "wore a patch over one eye to keep it dark-adapted outside." Should a battle break out and the pirate had to shimmy below, he would simply switch the patch to the outdoor eye and he could see in the dark right away—saving him 25 minutes of flailing his cutlass about in near blindness.