Extraretinal photoreception in mammals?
Its pretty well established that there are photoreceptors in cells besides the cones and rods in the retina of the eye.  Humans and most animals have four light receptor genes known (so far).  In addition to Rhodopsin - there are the short, med and long wavelength opsin genes.  

While they are mostly expressed in the retina of the eye, they can be found in many other tissues as well. The first image from GeneAtlas below shows the relative amount of RNA found for short wavelength opsin in a variety of tissues - its expressed in immune and nerve cells ( cyan and forest green respectively) relatively well too.   This might imply that neurons are light reactive.  Compare that to that of medium wavelength opsin, which is primarily much more common in the retina.  




This is not a psychosomatic effect.  Light receptors in the skin are known to help with seasonal affective disorder - shine a bright or blue light behind your knees.  These receptors are not connected to optical nerves, and so you don't get any image from them, but the information can affect your biochemistry anyway.  

The idea of unconscious receptor inputs from other parts of the body probably applies to many sorts of receptors.  This last year there has been a great deal of interest in taste receptors which are expressed in the gut.  They can taste sweetness and other flavors a second time and register the gustatory response in the brain.  Its not a conscious input, but it registers in the brain in MRI.  

Why would evolution do this?  It seems to me that this is a new way of looking at the individual life of a cell and makes a good deal of sense.  If every cell has all the genes' DNA why wouldn't a little bit of receptor expression be found in any cell which could use the information?  The conscious processes of the brain probably only take in a small fraction of the information that is sent in and there maybe hundreds of other such senses from various parts of the body to integrate, only a fraction of which we are aware of.  

In addition, there are probably lots of cases where receptor signals are only used locally by cells that are sensing their local environment.  It really doesn't make sense that the individual cells must blind themselves.  Individual bacteria and fungi have scores of receptors.  It makes sense that cells that are part of an organism would have as many or more senses as well.