As a result of these experiments, researchers realized that this is the mechanism by which nighttime lighting increased cancers in nighttime shift workers. Nighttime shift work disrupts the normal rhythm of the circadian clock, which suppresses melatonin production; suppressed melatonin levels correspond to decreased resistance to cancers in tissues with melatonin receptor sites and to increased growth rates in tumors with melatonin receptor sites. Colorectal cancer is probably the type of cancer affected most by melatonin levels, because the same modern lifestyle that causes exposure to nighttime lighting also includes a high-fat, low fiber diet that is a known factor in rectal cancers.

The two final piece of the puzzle fell into place when researchers also determined that: (1) mice exposed to very low levels of light, even during nighttime sleep, also had higher cancer growth rates; and (2) people who are completely blind have lower rates of the same cancers studied than people who are legally blind but able to see small amounts of light.. All of this finally explained why people who sleep just as many hours during the days as people who sleep at night still suffer from more cancer, simply because their sleep is during the daytime when their bedroom are less dark than at night..

The hypothesis suggested by the experiments is that we are already at risk of colorectal cancer because of our diets, and therefore, decreased resistance to those types of cancers only makes us that much more at risk. Likewise, breast cancer is also very common, already. Because breast tissue is sensitive to melatonin levels, it is another cancer that increases dramatically when melatonin production is suppressed.

Implications for Human Health:

The most important implication of the experiments into the relationship between...

Even very dim light during sleep, (such as from a television picture), disrupts circadian sleep- wake cycles. It turns out that T.V. is particularly harmful, because blue wavelengths of light have the greatest effect on circadian rhythms. Similarly, modern fluorescent light is bluer than yellow light from incandescent bulbs.
Ideally, we should seal bedroom windows from any stray light source so that we sleep in total darkness. Use of sleep masks is another option that may be easier than completely darkening rooms against very bright daytime sun. Apparently, it is the difference between totally dark and mostly dark that causes the health problems for everyone, regardless of when they sleep.

Another problem is that shift workers often rotate shifts with each other, so that all of them have a chance to work during the days on certain days or weeks (or months, as the case may be), and that this alternating between sleeping at night and sleeping during the day is another important factor. Consequently, it is healthier to work at night all the time (or at least for longer periods of time before rotating) than to switch back and forth between days and nights more often. Ultimately, the research suggests that lighting changes would improve the health for many people besides nighttime shift workers: the same principles and solutions apply to nursing homes, and even to streetlights, because they are responsible for stray lighting into our bedrooms at night as well.