A team of neuroscientists has worked out how newly discovered light sensors in the eye, called melanopsin molecules, detect light and communicate with the brain. These light sensors are a small number of nerve cells in the retina that are not used for seeing images. Instead, they monitor light levels to adjust your body's clock and control constriction of your pupils
Researchers first tested the light sensitivity of these cells by flashing light at the cells and recording the electrical current generated. They found that these cells are relatively insensitive to light, and are 5,000 times sparser than other light-capturing molecules used for image-forming vision.
However, the cells trigger a very large electrical signal that goes directly to your brain. The signal is very slow, so it is not intended for detecting very brief changes in ambient light, but slow changes over time.