Word for someone who sleeps during the day

People are sometimes said to be a morning person or a night person. I also know that the term for something related to night is "nocturnal".

Is there a specific word for a person who sleeps during the day and is active at night? I’m looking for an elegant word for night owl.

Your question already contains the best adjective: nocturnal refers to being active at night and sleeping during the day. I don't know about elegant, but this is certainly the most technically-correct word you're going to find. Unfortunately, it doesn't have a noun form, but that can usually be fixed by rearranging your sentences a bit.

Sally was a night owl who was always late to early-morning meetings.

Sally's nocturnal tendencies made her chronically late to early-morning meetings.

I do want to note that in most contexts, I wouldn't interpret either night owl or nocturnal (when applied to people) as explicitly sleeping during the day and working at night. Both words just imply that, left to her own devices, Sally would prefer to go to bed late and get up late, where "late" could have different definitions for different people. If you need to make it clear that someone sleeps during the day, then either you need to say that ("sleeps during the day") or refer to them working the graveyard shift (which admittedly is not the most elegant of phrases).

Carly Simon sings, "I'm a night owl, honey, sleep all day long." Wiktionary defines night owl by "(idiomatic) One who stays up late at night or goes to bed late."

It's not unusual to call someone a night or nocturnal person - normally it just means they "perk up" more in the evening, and don't tend to go to bed early. There's also crepuscular (of twilight, dusk), but that's not often applied to people. But as OP apparently knows, the standard term in common parlance is night owl.

It's not got quite the same meaning, but here in South-East England it's not unknown for parents to refer to their teenager as a troglodyte - meaning he spends all day in the darkened "cave" of his bedroom with the curtains drawn, only coming out to eat and socialise in the evening/night-time.

OP should definitely avoid referring to a female as a lady of the night, since that's generally understood to mean a prostitute.