Meaning of ‘Drift off’ Phrasal Verb in English

Reading a book in bed often makes me drift off quickly.

During the boring lecture, some students began to drift off.

I like to listen to soothing music to help me drift off to sleep.

He tried to stay awake during the meeting, but he kept drifting off.

After a long day, I started to drift off on the couch while watching TV.

As I lay in the hammock, I couldn't help but drift off under the warm sun.