night owl 
noun informal
A person who is habitually active or wakeful at night: dedicated night owls may retire to the late bars...
- I had a lovely conversation with his mother, my grandma Susie, on the phone today, and, thanks to the magic of instant messaging, just now wished my mum (who is a night owl like me) a happy mother's day.
- ‘Because of the nature of my security business, I am a bit of a night owl and my company's name is Night Security so I combined the two to come up with the title for the new CD,’ he smiled.
- There is also a worthwhile network of night buses, especially in the centre of London which for the night owl are both frequent and comprehensive enough to be of real value.