How it works  Spectrum
  The light spectrum visible to human eye is in the range of 380-780 nm.  There are two types of photo-receptors in the human retina responsible for vision, and they are known as rod and cone.  Rod is for seeing things at night time, and cone is for seeing things during daytime.  In 2002, a newly discovered third type of photo-receptor called Melanopsin, and it is not for seeing things but for regulating Circadian Rhythm.  

  The biological effect of Melanopsin is most sensitive to blue light of about 460 nm wavelength1. Cool white light contains higher proportion of blue wavelength than other colors, and blue light is a potent suppressor of Melatonin.  So when Melanopsin detects more blue light, lesser Melatonin is produced and thus making you feel more awoke.  When you are exposed with lesser blue light, more Melatonin is produced and thus making you feel tired.  
  1 Brainard et al., (2001) Action Spectrum for Melatonin Regulation in Humans: Evidence for a Novel Circadian Photoreceptor, Journal of Neuroscience, 2001

2 Bowmaker et al., (1979) Visual pigments of rods and cones in a human retina, J. Physics (1980), 298, pp. 501-511
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