He has a blog post about sleeping on night trains. He doesn't pay extra for a couchette or compartment, he sleeps in a normal compartment laying down over 3 seats or on a wide luggage rack with an air mattress.
Sleeper trains need a supplement and are often booked out in advance, so he sleeps on the night ICE trains (https://leben-im-zug.de/howto-nachtreise-im-ice/). These are regular trains with standard seating only, all lights on and announcements for the (frequent) stops at normal volume. He mentions that he sleeps on an air mattress on the floor.
