night train Venice to Rome

The only night train without train changes departs Rome at 22:35 (10:35pm) and arrives at Venice Santa Lucia station at 5:20 in the morning, for a total of 6h and 45 min of travel time. Not sure if it's worth it. Much easier to take a high speed Freccia train (high speed trains don't operate during the night). Last Freccia to Venice in the evening departs from Rome at 19:55 and arrives at Venice S.L. at 22:35 (3h 40min).
The first one in the morning departs Rome at 6:55 and arrives at 10:35 (also 3h 40 min)

thank you: that's very helpful since I was only able to find listed trips which were over 6 hours long on the RailEurope site... the high speed definitely is the way to go. is it necessary to purchase those tickets much in advance?

you can buy Freccia tickets on the spot at the station. Seats rarely if ever sell out for a train. If it happens (never happened to me) just select the next train (trains are frequent). However if you purchase in advance on line (up to 119 days prior max) you may have a chance to buy a Supereconomy or Economy tickets (they sell out fast, so go on line at 119 days prior). A caveat, however. Supereconomy and Economy tickets offer substantial discounts, but also come with restrictions. They are non refundable and non changeable. That means if you don't make that specific train at that specific time and date you purchase the discounted ticket, you lose all your money on those discounted tickets.

Roberto gives great advice. Wanted to note that if you are not finding any high speed trains for your specific day, the schedule is likely not out yet. Wait until it is a little closer, check back to see if the schedule is up. To get a rough idea, choose a similar day in the next few weeks (for example, if you'll be traveling on a Friday, choose Friday 5 April) and see what the schedule looks like. It shouldn't change dramatically and that can help you start to plan your journey. Like Roberto, I wouldn't take the night train - I'm not sure how well rested you'd be. Spend the night, take the earliest train (you could leave as early as 5 or 6, and it takes between 3:12 and 3:40 to get to Rome).

Beth, There's really no reason to take a night train from Venice to Rome, as that's a relatively short trip via Freccia or Italo high speed trains. The trains run frequently on that route and travel time is only about 3H:40M. If you pre-purchase tickets online, significant savings are possible, however that will lock you into taking a specific train and departure time. With nights trains, you'd also have to pay extra for Couchettes. The travel time is about 6 hours with the Intercity Notte trains and they make frequent stops, so you may not get much sleep. A day train would also allow you to enjoy some of the great scenery through Tuscany. Happy travels!