Melanopsin pigment in the eye could be more light sensitive than the pigment that allows for night vision, rhodopsin.
A two-year study using supercomputer clusters has shown that melapnopsin, a photosensitive retina pigment that senses non-visual environmental light, is highly receptive to light changes and allows the nervous system to synchronise with the day/night rhythm.
It is now known that the human retina contains not only photoreceptor cells, as previously assumed, but photosensitive pigment as well.
Read more at Bowling Green State University