Its "auto" vacuum which means it runs (nearly) continuously looking for things to do.
I suppose that in theory it could operate in a kind of "wait/notify" mode where every DDL against the database wakes the daemon otherwise it stays asleep. I recall some work being done a number of years back on making the system more energy efficient during no-activity periods - but there are trade-offs to consider here for which I am not up-to-speed on the specifics.
It would be interesting to see the timestamps on those log entries...
