In a study conducted on mice, scientists found that in mice retina there are photosensitive retinal ganglion cells that express the photo pigment melanopsin which has a great impact on sleep induction based on a response to light. Wanting to test the specific needs of the light they formed a hypothesis that the blue light which had the closest frequency to the receiving ability of the melanopsin would cause the fastest sleep induction. However when testing using green, blue and violet lights, they found that green caused the mice to sleep in 3 minutes versus the 18 minutes of blue light and 10 minutes of violet. All were tested at the same time of night. They found that green lights produced the maximum amount of rod opsin, and melanopsin responded better to the higher amount of rod opsin causing the mice to fall asleep faster.