How did we develop a circadian rhythm?


How does our body know we have a 24 hour day/night cycle?

In: 6

A significant component of it is visual. There are cells in your eye called intrinsically photoreceptive retinal ganglion cells (iprgcs) that are sensitive to blue light. These cells stimulate a certain part of the brain, the suprachiasmatic nucleus (scn), that regulates sleepiness. When there is no blue light, ipgrcs tell the scn, which causes the pineal gland to make melatonin. Melatonin makes you sleepy.

The body naturally have a circadian rhythm which can be measured by the hormone melatonin. When you have too much melatonin the body starts increasing the rate it breaks down the melatonin, but once the melatonin levels have returned to average the body still continues to break it down. When the melatonin levels gets too low the body starts increasing the production rates, but similarly the production rates is still high as the melatonin levels return to average. This entire process of breaking down and producing melatonin takes roughly 24 hours, usually longer.

There are also a number of things which break down or produce melatonin. We have not mapped them all. The most important however is the eyes which break down melatonin in response to light, mostly blue light. This allows the natural circadian rhythm to sync to the day and night cycle over time. But there are plenty of other things impacting your melatonin levels such as meals increasing melatonin and also your activity levels.