Take a look at the night train details on the Deutsche Bahn website. There's one departure that involves no changes (good), but it makes 15 intermediate stops, 10 of them after midnight (not so good). Saving one night in a hotel wouldn't be worth it to me, because I'd be dead on my feet the first day in Krakow.
Allowing just one night in Venice means only a few hours there. You probably wouldn't have time to get beyond the most tourist-thronged streets, which would be a shame.
As for the time required to change cities:
- The packing step can be quick.
- Hotel check out can be quick or a bit slow if others are leaving at the same time. I often settle my bill the night before so I don't risk a delay when I'm heading to the train or bus station.
- Getting to the train or bus station: that varies a lot, depending on the location of your hotel. Budget hotels are sometimes a bit obscurely located. I've often had a walk of a mile or more (close to half an hour pulling a suitcase). Some of your cities are quite large and might involve taking public transportation to the bus/train station; those schedules are unpredictable so require padding. In Venice you may need to take a vaporetto to the train station.
- Time at the departure station needn't be long if you've already bought your ticket (if not, good luck to you in Italy, especially), but you do need to find your platform, and some of the stations are huge. I'd want 15 minutes at even a small station, more at a large one.
Total of the above: Minimum of 45 minutes with an ideally-located hotel (which will be unusual). Maximum of maybe 90 minutes. Oh, you're flying that leg? Add at least two hours; maybe more, depending on how far out the airport is.
At the other end you need to find your way out of the train/bus station and figure out where you are in the city (which way is north? which direction takes you toward your hotel?), get to your hotel, complete the check-in process, drop your bags in your room, ask the desk staff how to get to your first stop, and get there. My first stop is always the local tourist office so I can pick up a city map, check on the opening hours of sights I know I plan to visit, and find out whether there's something special going on that I'd like to see. Time required varies with the proximity of your hotel to the train/bus station and to your first sightseeing stop.
I figure at least an hour lost on the arrival end of the trip. If you're flying, add perhaps 45 minutes extra, even if the airport is quite close to the city.
So I assume I'll lose 2 hours plus the transportation time if I change hotels via bus or trains, much more if flying. This is why we typically say that any change of city costs at least half a day. Also, it's difficult to quantify how much time you lose by constantly being in a new city and not immediately knowing where the closest market/store/subway station is.