In a scene where there is absolutely no way of telling whether it is day or night — say, when a character is inside a meat locker in the bowels of an underground nuclear bunker and has nothing but a Zippo to see by — should one write…
- INT. MEAT LOCKER – DAY
- INT. MEAT LOCKER
This is assuming it is daytime above, and realizing that there is no problem shooting the scene in the daytime.
There’s no universal consensus, but I vote for the former. Even if you’re not seeing the sun or the moon, there’s generally a “daytime” or a “nighttime” vibe. The crew of a space station, for example, needs to sleep at some point. Call that night.
Incidentally, don’t have the alien attack then. Kind of cliche, in my humble opinion.