Android: While many smartphones are OK at detecting changes in ambient light, most have a minimum brightness setting that can still be a little too luminous for your surroundings. Enter Night Mode.
Night Mode allows you to override your system's brightness level and turn your screen down to levels you might not otherwise reach. In fact, you could reduce your brightness to nothing, although it's not advisable. The service will run in the notification shade, so it's easy to turn it off when you need to crank the light back up.