It's now known that the body clock is controlled by a tiny pea-sized organ in the brain called the suprachiasmatic nucleus, or SCN. This tiny region commands a chain of chemical and nervous instructions that ripple through the body, controlling how each organ and tissue functions over the 24-hour day.
The other part covered is our perception of time. It's a constant, but we've all had moments where time seems to fly or drag on forever.
Psychologist Dr David Eagleman, of the University of Texas, recently set out to nail this assumption, and a BBC film crew was there to record it. He asked volunteer Jesse Kallus to perform a terrifying backwards free-fall of 33 metres.
If the anecdotes are correct, Jesse's perception of time would be slowed by the terrifying experience. But how could one monitor such a thing?
To test it they created a wristwatch type device with a flashing screen that showed random numbers and flashed so fast no normal person would be able to read it. Jesse then had to try and read it while falling. He repeatedly managed to read the screen while falling, which proves that time can indeed stretch and compress on a very personal level for each of us.