Meaning
A face with closed eyes, an open mouth and a little “zzz” floating above it, sound asleep. It stands for sleep, tiredness or boredom so deep it puts you to sleep.
The literal use is simple: “Heading to bed early tonight 😴” It also says you are exhausted after a long shift. The bored meaning is the risky one, since sending it in reply to someone’s long story or a colleague’s update suggests they are dull. Save that use for friends who know you are joking.