intrinsically photosensitive Retinal Ganglion Cells
in your eyes..........
They play a major role in synchronizing
circadian rhythms to the 24-hour light/dark cycle,
providing primarily length-of-day and length-of night information
~they are easy to fool~
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even in 100% BLIND people
light enters the brain to stimulate it via these receptors
read more here
http://www.kurzweilai.net/study-with...1f6c-282118213
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