The lenses are very powerful. When sunlight shines through them onto the LCD screen, the screen is burnt very quickly. It's just like using a magnifying glass to burn wood or paper.
Normal ambient light shouldn't be a problem, but never leave the headset where the sunlight can shine directly onto the lenses.
i5 6600k - GTX1060 - 8GB RAM - Rift CV1 + 3 Sensors - 1 minor problem
Dear Oculus, If it ain't broke, don't fix it, please.