Of course, we all know scene headings should include DAY or NIGHT.

But what about when several of your key locations throughout the script are windowless rooms?   A submarine underwater, or inside a bank vault, for example.   Is Day/Night even necessary?   It seems to add unnecessary clutter.

I notice on the Apollo 13 screenplay, for Int. Mission Control there's never Day/Night assigned to it.

I'd love to hear your thoughts on whether Day/Night can be dropped from those scene location headings without rankling readers!  Thanks!

Hi Brad, welcome to SS.
As you're probably aware -

The Day & Night are usually reserved as a logistical factor for production but they also evoke for a reader.

Space never has a Day/Night, far as I know.

Alien screenplay, for example:

FADE IN

        SOMETIME IN THE FUTURE:

        INT. ENGINE ROOM

        Empty, cavernous.

        INT. ENGINE CUBICLE

        Circular, jammed with instruments.
        All of them idle.
        Console chairs for two.
        Empty.

        INT. OILY CORRIDOR - "C" LEVEL

        Long, dark.
        Empty.
        Turbos throbbing.
        No other movement.

        INT. CORRIDOR - "A" LEVEL

        Long, empty.

For your Submarine heading - INT. SUBMARINE would do, imho, and you could do similar to above. Best keep in mind from a lighting perspective:

https://www.quora.com/Do-submarines-have-different-interior-lighting-for-day-and-night
Apparently lights are different for day/night in a sub.

You might use a Super after description with where they are (what ocean) and the time perhaps?

Perhaps indicate looking at an internal clock 2300 Hours     or write in a Super: Twenty-Three Hundred Hours.
Cutting to outside shots, (if you do) will indicate Day or Night.

As far as your windowless location, or Bank Vault, or mine shaft, cave, or train tunnel, I'd leave it off and only consider it if cutting same time to an outside location. If for example you cut to a rescue team in another location readying to pull someone out of that Cave it might be DAY, but it's always going to be lit the same way in the cave, so I wouldn't bother. I'd bother more with atmosphere.

In The Descent 2

INT. CRAWLSPACE - DARKNESS
Tight. Claustrophobic. The team wriggle on hands and knees as
the tunnel tapers ever tighter. Rock bears down above.

It's a good question.
But yes, it can be dropped and no rankling should result.  

Thanks for your reply and thoughtful answer!  Very helpful.  
And thanks for being the Admin!

Quoted from BradH Thanks for your reply and thoughtful answer!  Very helpful.   And thanks for being the Admin!

You're most welcome, Brad! Hope to see more of you.
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Everything Libby said - plus time context.

It may matter for our characters and readers if it is day or night.

e.g.,

If Dave yawns and stretches - and it's day - that means something different if he does the same thing at night.

Also time context.

DAVE AT WORK - DAY

DAVE AT HOME - NIGHT

implies a passage of time not implied in:

DAVE AT WORK

DAVE AT HOME

Here’s a follow up question. Does anybody use morning afternoon evening anymore. I know theyre in trottier’s bible and are one of the defaults in final draft.

Reason I’m asking. I use to use them - then i switched to day / night. But i felt i lost continuity. Let’s say story goes from afternoon to next morning. I know you can clarify in action but that adds lines.

Seen both in scripts (and a lot of others that i have no clue about.

I use morning, dusk, and Dawn as well as night and day