The circadian rhythm is the cycle that dictates what your body does when: secretions, sleep, awake, temperature fluctuations, etc; but most importantly, when it does those things.
A circadian backflip is something shift workers and people stuck on night shift experience, when they wake after dark, and go to sleep after dawn.
Person 1- "Where's Cornelius lately? I haven't seen him in months!"
Person2- "Me neither. Since he started his new job he's had a full-on Circadian Backflip. He looks just like Bela Lugosi now!"