If you're cracking open an iBook, tap the font button in the top corner of the screen (the same "aA" button as in Safari) and tap the darkest circle for white text on a black background. There's also an Auto-Night Theme setting that automatically turns on night mode if iPhone or iPad's ambient light sensor thinks it's dark enough.
Got the Kindle app? Tap the "aA" button in the bottom corner of the screen and tap the big Black button for white text on a dark background. The Kindle app also has its own screen brightness slider that works independently of the main iOS brightness setting, perfect for making the screen ultra-dim. (I typically drift off about five minutes after starting to read a Kindle book at the lowest brightness setting and with the black background on.)
Many other popular iOS readers have their own night modes, too. For example, you can tap the "aA" button in Instapaper to change the background setting; same thing in Reeder (the excellent iOS RSS reader). For Pocker, tap the three-dot button in the bottom corner of the screen, then tap Display Settings.
Reverse the colors on the screen
Unfortunately, not all reading apps have a "night mode," including some of the most popular newspaper and magazine apps (and yes, I'm looking at you, New York Times). You'll also strike out if you're looking for a night-mode setting in the Mail app, meaning you'll be lighting up the whole room while checking your inbox.
iOS's Invert Colors setting results in groovy-looking images, but it also makes for a handy, de facto dark mode.
There is a way, however, to be a considerate late-night iPhone or iPad reader even when there's no dark-mode feature in sight.
Tap Settings > General > Accessibility, then toggle on the Invert Colors setting. When you do, all the colors on your iOS display will reverse themselves, resulting in some very groovy visuals on the home screen and in your photos albums.
Neat, but beyond its novelty value, the Invert Colors setting also acts as a de facto night mode. Jump to your Mail inbox, the New York Times app, or any webpage in Safari that doesn't support Reader mode and you'll see what I mean; yes, we're talking white letters on a dark background.
Bonus tip: You don't need to jump through three iOS settings screens to get to the Invert Colors toggle. Instead, just set up a shortcut. Tap Settings > General > Accessibility, scroll all the way down and tap Accessibility Shortcut, then selectInvert Colors. Now, just triple-tap the Home key to turn the Invert Colors setting on and off.
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