For a long time, it has been thought that the circadian rhythm is controlled by light, and while this is true several misunderstandings have existed in the past.
In the eye, there are the commonly known cones and rods which allow us to see. Many would assume these rods and cones are what determine our ability to keep track of the day and the night, and are what communicate with the circadian rhythm. The truth is that rods and cones do not communicate with the circadian rhythm. Cones and rods are photoreceptive cells that communicate to Midget Cells (80% of the retinal ganglions), to Parasol Cells (10% of the retinal ganglions) and Bistratified Cells (8% of the retinal ganglions). None of these retinal ganglions send information to the circadian rhythm (suprachiasmatic nucleus).
The cells that DO send information to the circadian rhythm are the Photosensitive Ganglion Cells. These cells play a major role in synchronizing the circadian rhythm to the 24 hours light/dark cycle, innervate other brain targets such as the center of pupillary control and they contribute to the regulation of melatonin hormone.