The rhythm of life!
So, firstly we can think of our circadian rhythm as simply an internal clock. It works roughly on a 24 hour cycle and all of the processes our body has to do each day (growth, eating patterns, repair, hormonal adjustment etc) are completed in this 24hour period, controlled by our clock. This is also the bit of us that gets confused by jetlag!
In order to keep our clock running to time and running efficiently, we have lots of external signs that influence it. Diet, temperature, medicines etc all play a role, but the key rhythm adjuster is light and dark. If you think of our ancestors, they didn’t have any clocks to tell them when they should and shouldn’t sleep, but they did have the sun. Not only can we see the sun and use it as a visual indicator (the sun is up, so let’s get up!), but there is a different part of our eye (not the bit we see with) whose job it is to signal to our brain whether it is light or dark. The really clever bit about this part of your eye is that it can even signal to the brain that it is light or dark outside when your eyelids are closed!