Can you even define "Nocturnal"

I even looked the word up to make sure it didn't have some sneaky letter in it.
So........
After an hour of re-putting and sending little missy back to bed, I was a bit exasperated, (we go through this every night) and I say

"Girl, you have got to learn how to go to bed"

and she replies, without missing a beat

"Mom, I'm nocturnal now, I sleep in the day so I can stay up all night!"
I can't remember the last time I laughed so hard, and I'm not sure, exactly, what I was laughing at. It may have been sheer exasperation or the fact that 6-year-old Isabel used "nocturnal" in such a complete and logical way. However, this does worry me for the teenage years!
I love you Missy!