Pupil Dilation--How It Works

Did you know there's an eye within your eye?  Tis true.  The top layer of your retina contains cells called photosensitive ganglion cells that both receive and process light separately from your normal rod and cone cells that form the images we see.  About 1-3% of the nerve cells cells within your eye (ganglion cells) have evolved to see light for a couple special reasons.

1. To set your circadian rhythms.  Ever noticed how difficult it can be to sleep with the sunshine pouring in your room?  I actually choose to keep my blinds open so that I wake up easier in the morning.  I had the effectiveness of this proved by travel quite a bit and having to deal with jet lag.  There's a reason for that.  These photosensitive ganglion cells talk to the pineal gland in your brain and, among other things, release or suppress hormones like melatonin which helps put you to sleep.
2. Cause your pupils to dilate or constrict!

These aforementioned cells use a photopigment called melanopsin that detects light that is essentially blue (460-484 nm).  Why blue?  If I had to guess, I'd bet that's the frequency that makes it through the eye lid the best!  

Very helpful flash representation!!

Really interesting article about pupil size, the appearance of attraction and therefore attractiveness.  

Other interesting questions: How is it that our pupils can dilate due to psychological reasons and we don't experience our vision brightening?  What is the brain mechanism that dims our vision during psychological dilation?

More:

http://en.wikipedia.org/wiki/Photosensitive_ganglion_cell

http://en.wikipedia.org/wiki/Pupillary_light_reflex
http://www.bettermedicine.com/article/pupil-dilation
http://www.carlyjcais.com/in-advertising-size-matters-pupil-size-that-is/
http://www.bettermedicine.com/article/pupil-dilation

Pix:

 http://www.images.missionforvisionusa.org/anatomy/2005/10/retina-answers.html

http://embryology.med.unsw.edu.au/Notes/eye.htm