Just to be clear, in case there are people actually wondering:
> Is it appropriate to smile at a coworker of the opposite sex? Or to instant message them about work late at night?
No. And no-no.
> Is it appropriate to smile at a coworker of the opposite sex? Or to instant message them about work late at night?
No. And no-no.
That's sufficiently bizarre that I assume you misread the original post... (I'm applying Occam's razor).
Likewise, if the on-call engineer is female, I'm going to text or even call her late at night if I need her help.
Thing is, when I randomly message someone at night, I don't expect a response, nor do I view my recipient unfavorably if they don't respond until morning or at all. Of course, it's different if the entire team is working late, but that's only happened a few times in the past 2 years I've been at my current company.