Translation:The youth of today does not sleep in the morning.
Nobody says "sleep in morning" in English. Sentences must be translated by their meaning, not word-by-word. You'll have a lot of trouble with non-romanic and non-germanic languages if you do that.
In French however, we would say "Dans LE matin" for "In the morning." I think each individual language just has its own semantics but clearly I don't understand those of Portuguese. Practice coming up...
I have the same problem - why is it 'de' which I understand to be 'of the' or 'from the' and not 'na'?
That makes no sense because na (em + a) has a definite article, and it refers to a specific morning, e.g. "na manhã do dia 21 de Outubro..." (in the morning of october 20th).
What I still don't get is when you use "de manhã". Why here, and when else?
I believe it's because the morning is something that occurs frequently. You use the same for afternoon and night. I think "na manhã" might imply a specific morning rather than mornings in general