"Night owl" people are people that stay up real late at night, maybe even all night. I used to be one.
I'm a night owl as well. I sleep better during the day. I have always been an insomniac.
I'm still not an 'early bird' though, which is someone who starts sleeping at an early time and wakes early.
I'm not either. If I have to wake up early, I'm a bit cranky.
In Scandinavia, early birds are referred to as "A-people" and night owls are called "B-people" hehe, so I guess that makes me a C-person.
Wonder what they would call someone who goes to sleep late and can't stay asleep? lol
Scientific researchers refer to early bird as "morningness" and night owl as "eveningness". It's called "diurnality" and "nocturnality". Interesting.
I have never heard those terms before. 
Me either, until just now. Some night owls stay awake until just past midnight but more extreme night owls stay awake until near dawn or right after dawn.
I'm kind of middle of the road, I go to sleep around 2 or 3