Many people like to use their phones or tablets at night before they go to sleep. However, the light from mobile devices can interfere with sleep. But not all light carries the same risk. Recent studies have shown that within the light spectrum, blue light can have the worst effect on sleep. A study in the UK showed that blue light actually can keep sleepy people awake. Tired drivers who were exposed to blue light became as alert as those who drank two cups of coffee. Blue light filters have been developed for mobile devices to help with sleep issues. Apps are available for Android devices that filter blue light on screens. Updates to the iPhone, iPad and Amazon Fire tablet have added blue light filters. Twilight is a free app that filters blue light on Android device screens. It can be set to turn on by itself. Or users can turn it on and off when they want. In the app settings, you can change the intensity of the filter, the color and the time the filter turns on and off.