http://www.newscientist.com/channel/life/dn14117-circadian-eye-could-be-key-to-insomnia.html

You might call it our circadian eye. A handful of retina cells sense light, not for vision, but instead to reset our body clocks each day. Killing off these cells in mice leaves their sight unharmed, but throws their clocks out of whack, two new studies show.