Have you ever been dozing on a plane as it begins its final descent, only to be jerked awake by a flight attendant asking you to raise your window shade so the bright sun can burn your retinas?If so, you're not alone—this is still standard procedure during takeoff and landing on most airlines. But the flight attendant isn't doing it just to punish you for asking for an extra baggie of corn chips. Like most of the little annoyances that make flying miserable, this is actually an important safety precaution that could save your life in the event of a horrible accident/fireball.Condé Nast Traveler reports that flight crews are instructed to make sure all the cabin windows are open for purposes of visibility in the case of an evacuation. U.S. regulators require that any aircraft can be evacuated in under 90 seconds, so any additional time required for the crew or passengers to adjust to the light could mean the difference between survival and a gruesome death.Allowing natural sunlight into the cabin lets everyone adjust in advance—a crucial step when you consider that most plane accidents happen on takeoff or landing. It also lets the crew see outside the plane during those critical moments. Furthermore, if the plane is evacuated, it will allow everyone on board to move away from the fuselage faster without the disorienting effect of stepping into bright sunlight. And trust me, you're going to want to move away from that fuselage.So next time a flight attendant politely asks you to open your shade, don't throw shade back at them. Thank them for doing everything they can to keep you from exploding.