Great Summary of Circadian Rhythms

, The Health Link at the Medical School of Wisconsin has one of the easiest to read and understand broad overviews of circadian rhythms that I have come across.  Circadian rhythms are important because many of us with SAD have body clocks that are very sensitive to light.  It's these circadian rhythms that help you wake up in the morning, go to sleep at night, and function on a day-to-day basis.  The circadian rhythms control your hormones, blood pressure and more.  The internal clock runs on a 25-hour schedule but can be reset by outside stimulus like light or alarm clock.

There is much reasoning out there that the lack of light and how it affects the body leads to SAD for some people.  Check it out, it's interesting!

Basic Overview: http://healthlink.mcw.edu/article/922567322.html