Not sure how often its used, but in iOS you can set accessibility settings so that triple-click home button will invert the screen. I have sensitive eyes and get migraines. Combine with the fact that modern designers are complete idiots (on every platform - Apple, MS, Google) and make everything blinding white with lots of empty space, as a result it can be very painful esp at night. So I used it a lot. I haven't found a way to do that yet in Android, maybe someone knows?