is "When are you going to sleep" unacceptable? can you translate "śpisz" as going to sleep?
Not really, 'When are you going to sleep' = „Kiedy idziesz spać”; „spać” vs. „iść spać” is more or less the same as 'to sleep' and 'to go to sleep' in English.
this is literal translation, and there is no "hidden meaning". Only clarification is that (ty) śpisz, is about one person only.
That is strange. I have done the "questions" unit twice, and "kiedy" wasn't introduced.
"When will you sleep"
if not Kiedy spisz
how then would it be translated
Is there also something like wyspać (się) for "sleep in (in the mornings)" or "sleep long enough to be rested"? (Like vyspať sa in Slovak or ausschlafen in German.)
Would you also use that to wish someone "Sleep well!" (Slovak Pekne/Dobre sa vyspi!) ?
Yes, "wyspać się" means "to sleep long enough to be rested" :)
I can imagine saying "Wyśpij się", but I wouldn't compare it to "Sleep well!" which is probably quite commonly said, I'd say it only to someone that is really tired/didn't get enough sleep the previous night.