Everyone has a slightly different circadian rhythm, but the average length is just over a day, at 24 and one quarter hours. The human circadian system is regulated by both external stimuli and endogenous clocks, both of which control daily changes in the pituitary, pineal, adrenal and thyroid glands. Of the external stimuli, none is more powerful than exposure to visible light, which signals the hypothalamus when transmitted through the eye.
Cyclical visible light and darkness are increasingly understood as powerful tools in regulating human health. Changes in hormones engendered by alterations in light spectra have been shown to have profound physiological and psychological effects. Studies have consistently linked shift work and nighttime light exposure to a variety of diseases, including breast and prostate cancer, diabetes, heart disease, obesity, and depression.
Though it is unclear at present exactly why artificial nighttime light exposure is so detrimental to human health, Harvard's Health Letter observes that it is known to suppress the secretion of melatonin, a hormone that closely influences circadian rhythms. Very preliminary evidence may link lower melatonin levels with cancer. Gradually altered circadian rhythms have also been shown to cause a prediabetic state, altering blood sugar and lowering levels of leptin, a hormone that controls how full we feel after eating.
Light comes in a variety of wavelengths and attendant colors, of course, and blue light (or short wavelength) has been shown to suppress melatonin levels more powerfully than other shades. A Harvard study concluded that blue light shifts circadian rhythms and suppresses melatonin almost twice as powerfully as green light. Another study utilized a control group that was exposed to bright light while wearing glasses that specifically blocked blue spectra. It concluded that the group exhibited melatonin levels similar to those found in subjects exposed to very low light levels.
Artificial light exposure, particularly in the blue spectrum, poses very serious questions about modern environmental concerns as they relate to human health. Many electronic devices are known to emit disproportionate amounts of blue spectrum light, particularly computer screens and many forms of energy-efficient lighting. Compact fluorescents and LED lights tend to produce more blue light than traditional incandescents, and while it isn't possible to change the physics behind these bulbs, there are certainly alterations that can be made in their construction. The coatings inside light blubs, for instance, can be changed to limit the amount of blue light projected.
There are other steps that a health-conscious individual can take in order to effectively maintain their circadian rhythm. Colored light bulbs are widely available, and dim red bulbs are best utilized for nighttime applications. Compared to blue and green light, the red spectrum exhibits far less power to interfere with natural biorhythms. Blue light blocking glasses are available as well, and could represent a valuable asset for those who work night shifts. Bright screens should be avoided in the hours before bed, though if you must use your computer, filters can be downloaded that will prevent your screen from emitting blue light in conjunction with the setting of the sun.
As our understanding of the body's circadian rhythms grows, it is also important to note that not all must be gloom and doom. Though nighttime exposure to artificial light does pose very serious health risks, the field of chronotherapy is also emerging from the same research. This discipline focuses on the circadian rhythm's particular effects on the immune system, analyzing how the body's immune response fluctuates throughout the day.
Although it is unclear what neuroendocrine control directly effects the immune system, receptors for neurohormones and transmitters are exhibited by immune cells. It is possible, therefore, to adjust the time of day that treatment for a disease is administered in order to optimize the immune response for the best results. Certain types of steroid hormone-dependent cancers in particular are susceptible to this kind of treatment. Other diseases that have circadian components, like autoimmune disorders, heart disease and diabetes, can also benefit from chronotherapy as an adjunct treatment.
Some scientists have asserted that developments in the field of photobiology over the last few years represent the most important light induced innovation since the invention of the light bulb. They predict fascinating times are ahead for chronobiologists, and those in light-related industries, as our improved understanding of circadian rhythms allows us to save energy and increase our performance and knowledge, while continuing to foster our health and well being.