Facebook Messenger users can finally rejoice: There is now an easy way to mitigate late night eye strain and save your battery — as long as you know how to unlock the hidden feature.

In the last few days, Facebook has started quietly rolling out a secret "dark mode" for Messenger that gives the app an inverted, white-text-on-black-background look. The color scheme makes it easier to read messages from the app in the evening.

Facebook has chosen to add this feature in a pretty unconventional way, too: To get the dark mode option in Messenger settings, you need to send someone the "crescent moon" emoji. This one: 🌙 .

After that, you'll see an animation of a bunch of moons falling down from the top of the app, together with the query to enable the dark mode option in the app's settings. Once you do that, you'll see the new option in the app's settings.

Dark mode, enabled. Image: Stan Schroeder/Mashable

A few days ago, this didn't work for me and a couple of colleagues at Mashable, but now it is working on both iOS and Android, so the feature must be gradually rolling out to users.

Dark Mode for Facebook Messenger didn’t work for me a few days ago, but now it works in both iOS and Android. pic.twitter.com/CZA9CzsgGh — Stan Schroeder (@franticnews) March 4, 2019

Dark mode comes with a warning that some parts of the interface might look odd, but everything looked pretty normal to me.

Besides aesthetic, another reason why you might want to enable dark mode in Messenger is battery saving. A black background is better than a white background for your phone's battery, though admittedly you'd have to use Messenger a lot to notice it.

Dark themes on mobiles are all the rage these days. A system-wide dark theme might soon be coming to Android; on iOS 11 and 12, you can already get a dark mode of sorts by going to Settings > General > Display Accommodations and choosing Invert Colors.