The blue light that emanates from your phone's screen is known to disrupt your sleep. So it's good news that Google has added a light filter to its Play Books app, which gradually tapers the amount of blue light used by your display as you inch closer to bedtime. All apps should have it. The feature, called Night Light, is hardly a new idea. But what is nice is how it ensures that the app displays text with increasingly red-green light, providing warmer pink-orange hues, as the evening wears on. The new feature is available on Play Books for both iOS and Android as of this week, and you just need to activate the Night Light option to make use of it. But as scientists recently argued, it would be good if all devices -- or at least apps -- had this feature as standard.