Why not use a blindfold for 5 days?
Are these people stupid or just trying to make everything more difficult than it seems?
A) Some light is likely to get in, and they would need to be in a mostly-dark room regardless to account for slip-ups. Even then, they wanted to get 100% darkness, not 99.5% darkness (by timeslice)
B) Ever worn a sleep mask or eye pillow? Your eye does different things when it's covered or has pressure on it (and a lot of pressure would need to be applied here, most likely). Having your eye "free" to look around (but having no source of light in the room) is likely to be physiologically different than wearing a dark blindfold.