It's no secret that the iPhone's glowing screen can mess with your eyes and throw off your circadian rhythms, resulting in a screwy sleep schedule. Enter Night Shift mode, which is basically a baked-in version of the f.lux iPhone app. It gradually and automatically changes the color temperature of your phone display based on the time of day, thus reducing the amount of blue light emitted from the screen for happier eyes and better sleep.
To set it up, first you have to update to iOS 9.3. Then go to Settings > Display and Brightness > Night Shift and select what time you want it to turn on. You can create a custom schedule, or set it to run from sunset to sunrise -- which it automatically detects based on whatever time zone you're in. And if you suddenly find yourself in some romantic mood lighting and want to enable it on demand, there's an easy shortcut via the Control Center: swipe up from the bottom of the screen and tap the sun/moon icon.