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: 🌙 .

Once you’ve done that, several moon emoji will rain down over your conversation screen, letting you know that the feature has been unlocked and can be toggled on or off within the app’s settings menu. Some users have reported instances of a pop-up window that will inform you of the new feature, though it appears that it doesn’t happen for everyone.

Dark Mode is available for both Android and iOS users, and if it doesn’t unlock with a moon emoji, make sure you are using the latest version of the app.

Facebook has said that Dark Mode for Messenger will be “fully rolled out in settings in the coming weeks.”