Circadian rhythm: It’s the reason you find yourself full of energy at a particular time of day (or night) and really dragging at another time.
Biological clock: Your internal timing device; usually a 24-hour clock. Your biological clock produces your circadian rhythm.
So, what does this have to do with your productivity and whether you’re a morning person or a night owl?
Well, your circadian rhythm is based partly on DNA and partly on external factors, such as daylight. “Larks” wake up and go to bed early; they tend to find 
themselves more productive in the morning. “Owls” rise and hit the sack later, preferring to get their stuff done in the evening or at night.
But researchers have found that most people hit their peak, creatively speaking, at precisely opposite the time they’re most productive. I, for one, agree with them.
I’m a morning person. When it comes to balancing my checkbook, editing my writing, or having to use my left brain, I perform much better between 6 a.m. and noon. However, the best ideas I’ve ever had for my writing–and ways to solve plot and character defects–always come in the middle of the night when I wake up to go potty or just after I slip into bed at night.
How does your circadian rhythm work with respect to your creativity?