Blacking out your bedroom is important for adequate melatonin production. Melatonin is a hormone produced by the pituitary gland (a little gland in the brain). Melatonin is considered to be a very powerful antioxidant and has been shown to have anti-cancer effects. It is also thought to have a host of other effects that are not well understood.
The amount of melatonin produced by the pineal gland varies widely depending on the amount of light that your body is exposed to. At one point this was thought to be directly related to how much light came in through your eyes; so eye masks were the solution. But it has been found that your skin cells also have photoreceptors in them (ie. they recognize light). One study that comes to mind: a small fiber-optic light that emitted about the same amount of light as a small candle was placed at the back of the knee of a man in an otherwise pitch-black room. It was found that he produced 70% less melatonin then without the light.
So what does this mean for you? In order to maximize melatonin production (or to prevent a drastic reduction in melatonin) you room needs to be pitch-black. The darkness test is this: once your eyes have adjusted to the dark room, hold your hand up directly in front of your face. If you can see your hand in front of your face, it's not dark enough.
While this is important for everyone, it is especially important for shift-workers. Due to the lack of a consistent schedule and often sleeping during the day, when rooms are often not even close to being dark and therefore much less melatonin is being produced, shift-workers have been found to have higher rates of breast, prostate and colon cancer. And those are just the cancers that have been studied so far.
See my next post for more detailed instructions on how to go about blacking-out your bedroom.