Watt's wrong with using dusk and dawn?
Nothing. Use the words you feel are best for your screenplay.
All my advice is from the perspective of a “reader” and producer in the
States reading a script by an unproduced writer. Don't get in the way
of a smooth read. Keep your format as simple and unobtrusive as possible.
Allow your writing to stand out rather than embellish the format.
Both of you see the comments here when writers ask for feedback. Even
non-professionals get hung up on format choices and mention them or
ask format questions. Imagine what professional reader have to deal with.
Covering 3 to 5 scripts a day we see all kinds of issues that make the us
think about the format and lose the story. A script that follows the format
allows the reader to concentrate on the story and characters.
What's wrong with using DAY and NIGHT?