Nod off
Definition 1 of 1
Pro Tip 1/3Unintentional Sleep
Used when someone falls asleep without planning to, often due to exhaustion. 
After the long drive, I nodded off on the couch.
Pro Tip 2/3Subtly Humorous
It can mildly amuse listeners as it paints a picture of someone's head nodding as they fall asleep. 
During the boring lecture, I caught myself nodding off.
Pro Tip 3/3Socially Embarrassing
Implies a potentially awkward situation, like falling asleep at work or in public. 
He was embarrassed after he nodded off in front of his boss.