I watched an interesting The Nature of Things episode recently called "Lights Out!" which was pretty eye opening on how dependent our circadian rhythm is on natural light cycles. For a long time scientists were puzzled as to why breast cancer was so much more prevalent in developed nations, after ruling out most of the obvious environmental differences they looked into unnatural light and BINGO, light can increase the risk of some cancers. They think its also bad for many other things like depression and obesity.
What they found was the body switches into melatonin mode at night, more specifically in the dark, and melatonin is extremely important for a host of things including our ability to fight cancer. The problem is artificial light (light bulbs, TV, laptop monitors, etc) keeps our body from switching to melatonin mode so instead of getting a natural 10-12 hours of melatonin production we get less, much less if you only sleep an average of 5-6 hours in the daytime. Anyway, there is much more info in the episode, it's Canadian programming so not sure if you guys will be able to watch but here is the link:
We explore how the type of light we are exposed to in the hours between dusk and bedtime can play tricks on our bodies and cancel the healthful benefits naturally triggered by the absence of light.