Someone who believe he/she can stay up the latest at night or often is only awake at night and sleeps in the day. However, his/hers belief is flawed because in reality he/she isn't staying up late at all.
Kai: "I stayed up until 9:00pm last night! I'm sooo nocturnal!"
Peter: "dude...9:00pm isn't late at all, you are such a nocturnal poser."