[i] Use ‘at’ with points of time in the day: at dawn
[ii] Use ‘in’ with parts of the day: in the morning (Longman)
They are saying that prepositions, at and in, are selected by their complements’ meaning: that is, exactly when or a period of time. So it has ‘at’ in [i], while ‘in’ in [ii]. And in the same line, it needs not to be a definite article in [i], it seems. Then, do we have to have the article in [ii] for subdividing a period of time, like as subdividing cake into pieces? If it’s the reason why ‘the’ is used, are there any other examples for the subdivision with definite articles?