I was pretty amazed to notice that my Mac actually wakes up for the sole
purpose of running a scheduled backup and goes back to sleep. Oh, but
Going to web with the issue helped, I found Apple support community discussion with topic "time machine wake up unwanted". I don't think my pre 10.9 did that. Anyway I can confirm that 10.9 does this rather stupid thing.
Luckily the discussion thread also offers the fix: "Time Machine won't wake up a Mac, unless another Mac is backing-up to a shared drive on it via your network, and the Wake for network access box is checked in System Preferences > Energy Saver". Definitely something for me to try.
It helped! I can confirm, that there are no backups for the time my Mac was asleep. Pretty soon I woke it up, it started a TM-backup and completed it.