QUESTION

[Will Rogers]

what's the French equivelent for a "night owl" (someone who stays up late at night)?

[Charles D]

Noctambule--according to a dictionary I have.

[Neil Coffey]

More informally you can also say "un couche-tard" (I guess the slight difference is that "noctambule" implies the person likes to go out and do late-night activities, whereas a "couche-tard" is simply somebody who regularly goes to bed late).