Night Light, also commonly referred to as night mode, limits the amount of blue light your display uses, which, in theory, helps you sleep better at night.
Right now, Night Light is in early testing in the Chrome OS Canary channel, meaning it takes a bit of work and tinkering to gain access to it. But in the coming weeks, the feature should make its way to the developer, beta and stable channels of Chrome OS.
If you're the type who loves living on the bleeding edge of tech and doesn't mind running into a few bugs, you can switch your Chrome device to the Canary channel in under an hour. First, you'll need to activate developer mode as outlined in this post. Then, follow the instructions in this post to install the latest Canary build.
Don't get intimidated -- the process is a lot easier than it looks. Just remember to move any local files on your device to Google Drive or some other service before you begin, as it does require a complete reset of your computer.
With Canary installed, click on the system tray to activate Night Light, then on the circular moon icon (although it doesn't look anything like a moon).
Right now, the feature lacks any settings to fine-tune the intensity or set up a schedule for Night Light to turn on or off on its own.