The Night Shift feature automatically adjusts the temperature of your display to the warmer end of the colour spectrum after dark. You might want to do that if you find yourself looking at your iPhone a lot before going to bed as it can help you get a better night’s sleep.
It’s not on by default so to turn it on, go to Settings > Display & Brightness > Night Shift. You can also set when you want it scheduled to run and the warmth of the display from “Less warm” to “More warm”
The effect isn’t subtle when it turns on: around sunset, your screen will fade to a much warmer colour. But studies and anecdotal use have shown that it helps make the iPhone easier to look at and can help reduce the impact that the phone’s bright, glowing screen has on your sleep and eyes.
In general, looking at your phone at night is a terrible idea as smartphone screens emit bright blue light so you can see them even at the sunniest times of day. But at night, your brain gets confused by that light, as it mimics the brightness of the sun. This causes the brain to stop producing melatonin, a hormone that gives your body the “time to sleep” signal. Because of this, smartphone light can disrupt your sleep cycle, making it harder to fall and stay asleep – and potentially causing serious health problems along the way.
Night Shift isn’t without problems, though. It can’t be used during Low-Power Mode, which a lot of users have switched on at night. Though this may change in upcoming iOS. Color accuracy is also a problem. Photo editing or trying to determine colors accurately late at night isn’t a good idea with it enabled.