I looked at the train station schedules. I'm taking a night train from Venice to Monterosso (Italy) and it says there are three "Chg". Does this mean that we will have 3 train changes on the trip ? If that's true, you'd pretty much have to stay awake the whole time in order to move from one train to another.
Unfortunately, that's exactly what it means. There are four overnight trains in all....the best may be one of the two routes using the Montecarlo (the sleeper train), which take 7 1/2 or 9 1/2 hours (depending on routing) and only require one change, at Genoa, at 4:15 am, after which you will have no sleeping accommodations. But that's about the best you're going to get, since the other combinations don't have any sleeping accommodations anyway....unless you don't mind sleeping sitting up.
In addition to Norm and Kent's comments, it is not uncommon to change trains in the middle of the night and wait 1-2 hours for the connecting train. You won't get much sleep on any run from Venice to Monterosso.
Oddly, I don't see a night train run from Venice to Monterosso with 3 train changes. The ones I see have 1, 2 & 4 changes. I'm curious to know what time your train departs.