3 AM, never fail to wake up.

Hi so, something Ive noticed and it never really raised any flags until it was consistent when i got my FitBit (it tracks  your sleep) is that i always wake up at 3am every single night, no matter how late, early, or tired i am. I just suddenly feel like i had my sleep and ready to go. I usually go to bed between 8 and 9 pm, but last year i had a later job, having me go to bed between 10 and 11 pm. The 3am effect was the same then too. 
Its strange, and google had nothing scientific to say. Anyone?