Thanks, Track.

I bought a ticket from a DB vending machine here in Berlin and it wouldn' let me upgrade to the sleeper section for some reason after purchasing the Dauer Spezial fare. The best that was available was the 4´person section.

I would guess to be somewhat hardened after years of riding sleepers in Europe, Russia and China so no big worries about getting some rest. And a shower can be easily organized too in Dusseldorf.

Yep, in the case you described could as well go to my fav Turkish fast food joint near the station and start the day with a lentil soup and flat bread.