St. Moritz to Fussen Overnight Train?

There are no overnight trains in this region. The fastest daytime connections take 7 hours.

Thanks Laura for the reply. So do you think the Glacier Express would be too difficult to fit in? We are staying in Fribourg doing a home exchange. Should we just go by rail directly from Fribourg to Fussen?
Thanks!

The Glacier Express splits. one half goes to St Moritz, and the other to Chur. If you do this detour, it is the Chur branch you want. From St Moritz you would have to double back to Chur to go north to Germany and Füssen. There are no overnight trains.

Füssen and all the other places in southern Bavaria are difficult by train from Switzerland. The rail lines run south from Munich and then are dead ends, because the Alps are in the way blocking the route south to Austria. You can't go west to east, you need to go north and then south again.

You could do this, from Fribourg south Visp, pick up the Glacier Express there. It is not worth going further south up the dead end valley to Zermatt just to come back again. Stay overnight in Chur.
From Chur go on to Füssen the next day.

You can look up times on your favourite train time website: https://www.bahn.com/en/view/index.shtml

You need to look at this on a map that shows rail lines
Swiss Rail map: https://www.sbb.ch/content/dam/sbb/de/pdf/freizeit-ferien/ferien-kurztrips-schweiz/internationale-gaeste/sts-geltungsbereich_en.pdf
Rail map for Land Bayern (state of Bavaria): https://www.bahn.de/p/view/mdb/pv/deutschland_erleben/allgemein/tickets/pdfs_laender-tickets/2015/mdb_190311_streckenkarte_bay_2015.pdf