I don't understand why they accept "é na madrugada" here and doesn't accept it when you have to translate "It is at dawn" to portugues, it's no sense. I can understand that there is some context or specific times where you can accept na or de but in duo lingo theydon't tell you about the context, you see only the sentence. So I don't think it's right to lose a heart for this!!!
We usually say "é de madrugada" when we really are at this: What time is it mom? -- "é de madrugada". "É na madrugada" for things that occur at that time or a specific dawn. "What time is this series on tv?" -- "é nas madruagas de sábado para domingo" -- "é na madrugada que os bandidos agem"