That's what it sounded like to me. So I typed that and submitted it. Then I realized, "that can't be right".
I have learned in Duolingo that if I hear something that I know is not grammatically correct, I should not type what I hear. I need to figure out what I should be hearing and type that.
"noctū" == "at night", "by night" (locative of time within which (or ablative of time within which, using an Old Latin ending for the ablative))
"in nocte" == "during the night" (found more in Late Latin than in Classical Latin)
"noctē" == "by night" (adverb)
So, from Old Latin, through Classical Latin, to Late Latin, Romans did not use *"in noctū", apparently.
OTOH, in Classical times, the Romans did use a bare "noctū", as in
"Noctū ambulābat Themistoclēs quod somnum capere nōn posset" (Themistocles used to walk about at night because [as he said] he could not sleep) [Cicero, in Tusc. 4.44]
Breakfast was more ientaculum/jentaculum I think.
Prandium, was "originally first meal" (prāmdeyom ), but the meaning seems to have changed to mean "lunch".
Note: in the root jento (ientō),
we find the roots of the Spanish "desayuno and the French jeûner/déjeuner.
Hello Mona, it is a lone word ablative so technically yes it is a noun. It comes from nox Latin for night which is third declension. You would expect "nocte" to be the ablative and nocte is a synonym for noctu. Ablatives function like adverbs in English. Translated as at night or by night.