To be honest, I think it unlikely that many of us were even thinking of using the word nocturn today.
We might know that a nocturne (with an e) is a work of art giving some impression of the night (or it can be a ripply dream-like piano piece); and we might know that a noctule is a night-flying bat; and that something nocturnal is active at night; and, given all that, we can probably take a guess that the word nocturn is something to do with the night-time.
And we'd be quite wrong.
A nocturn is any of the sections of the Roman Catholic church service called matins.
Yes, that's matins as in morning service. From the Latin mātūtīnus, which means of the morning. From Mātūta, goddess of the dawn.
Personally, I think that's just perverse.
Sunday Rest: nocturn. This word comes from the Latin word nox, which means night. (The service of matins can be very early indeed in some religious communities, but I'm afraid that's just transferring the essential difficulty elsewhere.)