"al" is confusing
I looked up the personal a on another site because it seemed to slip in here without explanation. What I gather so far (and it is not at all clear) is that if the object acted on is a personal reference the a is added.
Why is it used in the sentence, "La niña duerme una vez al día?" The subject is personal but that does not explain the 'a'.
"Al" is a contraction of "a el". The sentence you note translates to "The girl sleeps once per day." If the sentence were "The girls sleeps in the morning", it'd be "La niña duerme a la mañana".