There aren’t any true “overnight” trains on this route. Most direct would be Venice-Brig-Kandersteg-Blausee. This takes about 6 1/2 hours. The best option with a “night train” would be Venice-Bologna, Euronight train to Brig, then Brig-Kandersteg-Blausee

You can see your options at http://www.sbb.ch/. (on the first screen, switch to English).

As you’ll see, it is a little less than a 7 hour trip, unless you pick the option that sits in a station for an extended period, which makes it a little less than 11 hours. Also, all of the options have a lot of vehicle changes between trains and buses, which would cut into your uninterrupted sleep time, and of course, you would miss seeing the scenery you pass at night.

I didn’t check the price, but years ago we never used a sleeping car; we just slept in our seats. Its up to you whether you can stand that.

Have you considered staying in Mestre rather than Venice; it has much better prices.