Even though leaving a fly mask on all night won't make your horse go blind, it's a lot like wearing sunglasses in the dark. For optimum vision, take the fly mask off at night, and don't forget to check his eyes and clean his face daily.
Unfortunately, mosquitoes can bite through some mesh. For extra protection, spray your horse or the inside and outside of your flysheet with a spray meant for mosquitoes.
Article is printer here with permission from Horseillustrated.com