In 2001, George Brainerd et al at Thomas Jefferson University in Philadelphia provided experimental evidence of a new circadian photoreceptor and determined the sensitivity of it in terms of light spectrum and intensity though as yet had not found the receptors themselves. http://www.jneurosci.org/content/21/16/6405.full.pdf+html
In 2002, researchers at Brown University reported experiments on non-visual retinal photoreceptors tied to the superchiasmatic nucleus (SCN) in the hypothalamus showing and discussing a relationship to melatonin production. http://isites.harvard.edu/fs/docs/icb.topic197607.files/Due_Wk_05_Oct_17/Berson_Phototransduct_retnal.pdf (change link to retinal)
In 2004, Revel et al at Rush University in Chicago described results of experiments that show that the circadian rhythm can be shifted with light exposure. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841985/
In 2011, Gooley et al at Harvard and University at Surrey performed a study on 200 individuals demonstrating suppression of melatonin production with exposure to normal levels of light in the evening before bed. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3047226/