Android's always-on display turns off most of the screen, besides a clock and set of notification icons. And the display constantly shifts to avoid burning out the screen.

It looks like iOS, on the other hand, just dims your lockscreen slightly? I can see why they'd have to get smart with how often they leave the display on. It'd be distracting while you're trying to sleep and sounds like a battery killer. In a way, Apple may be using the phrase "Always-on display" more literally than you'd expect, even if it is less practical.