The third and final mode is sleep; just before you hit the sack, press and hold the display to activate sleep mode, which is indicated by a small moon icon. (phoneradar.com)
Multi-frame low light mode is automatic and enabled by default, and is denoted by a yellow crescent moon icon on the main display. (androidheadlines.com)
If «Do Not Disturb» is enabled on your iPhone, you'll see a crescent moon icon on the right side of the status bar, as pictured below. (howtogeek.com)