I checked some examples, some are singuar, some are plural, but seems to be of the same type.

Examples from my Russian-Chinese Dictionary
по году не писал писем
сидеть над книгой по целым дням
Не видал его по целым неделям

In my Russian-Chinese dictionary it says on по + dat. in this meaning: "during the whole period, meaning the duration/length of time the action lasts".

I wonder if по утру means something like "the whole morning", thus the above sentence "you cannot be without the daybreak for the whole morning."

But утром does not always mean something that lasts the whole morning, like утром я чищу зубы.

Am I correct?