Industrial Revolution, Most People Worked

¶ … Industrial Revolution, most people worked and carried on their daily life activities exclusively in the daytime and slept at night, primarily because without artificial lighting, there was no other choice. Once kerosene, gas, and incandescent lighting became widely used during the nineteenth century, people were able to make more productive use of nighttime hours, and 24-hour production became a possibility.

Artificial lighting has become so common, that in modern societies, we take nighttime sporting events, all-night restaurants, and 24-hour casinos for granted. Certain industries run 'round the clock, with rotating shift workers on duty at all times.

When medical researchers realized that certain illness rates, such as colorectal cancer and breast cancer, are much higher among nighttime shift workers than in the rest of the population, they began searching into the reasons for the link between nighttime shift work and those higher illness rates. One of the most significant discoveries is that exposure to light during the nighttime hours triggers a response in specific nerve cells within the eye that, in turn, initiates a mechanism that makes us more susceptible to disease. This is a result of disruption in the normal circadian rhythm of sleep-wake cycles, because normal production levels of the hormone melatonin within the pineal gland is regulated by the circadian rhythm. Melatonin is known to have certain anti- cancer roles. Therefore, inhibition of melatonin production naturally increases cancer rates of those types. The implications are that sleep deprivation is not just a matter of how many hours we sleep, but also a matter of sleeping in complete darkness and avoiding schedules that interrupt the circadian rhythm.

The Link between Nighttime Lighting and Cancer:

When differences in cancer rates between daytime workers and nighttime shift workers first became apparent, researchers designed experiments to investigate any possible causal relationships between those observations and the identifiable differences between daytime and nighttime working factors. Among the elements of nighttime work considered was the factor of regular nighttime exposure to artificial lighting.

Since the two diseases noticed to increase most among nighttime workers were colorectal and breast cancers, researchers devised tests using cancer cell implantation in laboratory mice. Those experiments involved implanting human colorectal and breast cancer cells in mice and then separating the mice into different test groups. Some test groups of mice were kept in normal lighting situations where they were alternately exposed to light and then allowed to sleep in total darkness; the other groups were exposed to various types and levels of light throughout the night in addition to regular daytime lighting. The results of these experiments and others suggest that, like humans, mice exposed to light at night developed much higher rates of cancers, because the tumors grew much faster in the groups of mice exposed to nighttime lighting.

Circadian Rhythm, Melatonin Production, and Cancer:

Melatonin is a hormone that plays a role in inducing sleep, and it is known to be produced by the pineal gland in the center of the brain, with its highest rates of actual melatonin production triggered during the early morning hours. Both types of human cancer observed to increase in nighttime shift workers are related to melatonin, because both colorectal cancers and healthy breast tissue have specific hormonal receptor sites for melatonin. Earlier research discovered that the eye contains special cells within the retina that are responsible for regulating the circadian sleep-wake cycle. The circadian sleep- wake cycle is what regulates melatonin production levels, and the cancers that increase in nighttime shift workers (and laboratory mice exposed to nighttime light) are sensitive to melatonin. 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 lighting issues and human health is that people regularly exposed to light at night, especially for long periods, should make a specific effort to darken their bedrooms. 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.