into night or into the night

Question

I thought of another question led by a question in an exam.

The paper is due next month, and I am working seven days ______ week, often long into______night.

A． a; the

B．the; /

C．a; a

D．/; the

I agree with the correct answer A, but it brings me to a new question whether it is appropriate to write the sentence as "The paper is due next month, and I am working seven days a week, often long into /(the zero article) night." Or maybe it is good to put the question in this way, which is what the difference is between "into night" and "into the night".

Answer 1

Long into night wouldn't be ungrammatical, but it wouldn't be idiomatic either.

The Ngram is quite interesting; it shows the prominence of one but confirms the occasional usage of the other, such as in this poem by Rhona McAdam:

The last days of the season she lives in a world
of steam and spice, bends her back
to the cadence of knife and board,
chopping the coloured flesh of the garden,
working long into night.

Had the test makers included a third option:

E. a; /

they would have had trouble justifying that as a "wrong" answer, unless the test directions specifically instructed examinees to pick the more common alternatives.