Lighting for the human circadian clock: recent research indicates that lighting has become a public health issue

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Abstract

The hypothesis that the suppression of melatonin (MLT) by exposure to light at night (LAN) may be one reason for the higher rates of breast and colorectal cancers in the developed world deserves more attention. The literature supports raising this subject for awareness as a growing public health issue. Evidence now exists that indirectly links exposures to LAN to human breast and colorectal cancers in shift workers. The hypothesis begs an even larger question: has medical science overlooked the suppression of MLT by LAN as a contributor to the overall incidence of cancer?

The indirect linkage of breast cancer to LAN is further supported by laboratory rat experiments by David E. Blask and colleagues. Experiments involved the implanting of human MCF-7 breast cancer cell xenografts into the groins of rats and measurements were made of cancer cell growth rates, the uptake of linoleic acid (LA), and MLT levels. One group of implanted rats were placed in light–dark (12L:12D) and a second group in light–light (12L:12L) environments. Constant light suppressed MLT, increased cancer cell growth rates, and increased LA uptake into cancer cells. The opposite was seen in the light–dark group. The proposed mechanism is the suppression of nocturnal MLT by exposure to LAN and subsequent lack of protection by MLT on cancer cell receptor sites which allows the uptake of LA which in turn enhances the growth of cancer cells.

MLT is a protective, oncostatic hormone and strong antioxidant having evolved in all plants and animals over the millennia. In vertebrates, MLT is normally produced by the pineal gland during the early morning hours of darkness, even in nocturnal animals, and is suppressed by exposure to LAN.

Daily entrainment of the human circadian clock is important for good human health. These studies suggest that the proper use and color of indoor and outdoor lighting is important to the health of both humans and ecosystems. Lighting fixtures should be designed to minimize interference with normal circadian rhythms in plants and animals.

New discoveries on blue-light-sensitive retinal ganglion cell light receptors that control the circadian clock and how those receptors relate to today's modern high intensity discharge (HID) lamps are discussed. There is a brief discussion of circadian rhythms and light pollution. With the precautionary principle in mind, practical suggestions are offered for better indoor and outdoor lighting practices designed to safeguard human health.

Section snippets

Summary

Studies now indirectly link exposures of shift workers to light at night (LAN) and higher incidences of breast cancer and colorectal cancers. One possible mechanism is melatonin (MLT) suppression by shift worker eye exposures to LAN. The pineal gland hormone MLT is normally produced at highest levels in the early morning hours, but is suppressed by open eye exposure to LAN. Rat studies by David E. Blask and colleagues show that MLT acts as a protective oncostatic agent that suppresses growth

Light and the evolution of the circadian clock

Since we are born into a world of artificial lighting, we do not give much thought to how light disrupts the normal 24 h circadian rhythms present in all organisms. Biological clock rhythms are found in all living things, even in organisms as simple as algae [59], [60], [61], [62]. Clock genes that time cellular functions were likely inherited from the Earth's first simple organisms over 4 billion years ago. The circadian clock is dependent upon dark nights that allow normal MLT production

Light pollution

Light pollution is produced by the use of ever brighter outdoor night lighting that is often used for the advertising of gas stations, convenience stores, and shopping centers. It is often justified in the name of “security”. Since the 1960s we have seen the increased outdoor use of HID lamps.

Light pollution is light that is not targeted for a specific task, is bright and uncomfortable to the human eye, causes unsafe glare to drivers and pedestrians, harms the biological integrity of

Receptor cell discovery

In year 2002, Brown University neuroscientist David M. Berson discovered a previously unknown function for retinal ganglion cells (RGC) [7]. Previously unknown connections and functions of a small population of RGC located in the base of the retina of rats were identified. Berson demonstrated that RGC axons connect to the circadian or biological clock center – the paired suprachiasmatic nuclei (SCN) – located within the hypothalamus at the base of the brain. The retinal-circadian light

Blue light photopigment

Melanopsin, a blue light sensitive photopigment, resides within the small population of RGC [42], [49]. The RGC are located deep in the retina of the eye beneath the rods and cones. These specialized cells are non-visual photoreceptors previously thought to only relay to the brain nerve impulses initiated by light striking the rods and cones. The RGC and their dendrites fire maximally when stimulated by blue light at wavelengths between 464 and 484 nm [7]. By comparison, the visual system of

Light at night, shift workers, breast cancer and colorectal cancer

Because, much of the daytime work force in the industrialized world is exposed to dim office lighting instead of bright sunlight, and because at night we are often exposed to bright lights with blue wavelengths instead of natural darkness, some researchers believe that adequate human circadian entrainment does not take place [58], [74]. Without a normal cycle of MLT inhibition and production responding to cycles of light and dark, human health may be negatively affected [5], [22], [25], [29],

Melatonin regulates and protects

MLT is a hormone that has sleep-inducing properties and regulates a balanced physiological state in humans [59], [60], [61]. MLT also has substantial free radical scavenging/antioxidant and anticancer activity in experimental systems [45], [60], [61], [63]. This knowledge has obvious relevance to human health [22], [26], [54], [60], [61], [66], [79].

The production and suppression of MLT is a circadian driven event. MLT is synthesized by the pineal gland, a pea sized gland located in the center

How much, how long, and what color?

MLT suppression by LAN will depend on the color of the light, its intensity, and the duration of the exposure of the eyes (both opened and closed) to the light source. A study by Hatonen et al. [43] found that only two of eight human subjects exposed to 2000 lx of white light for 60 min with eyes closed between midnight and 2:00 AM showed lowered MLT levels. The authors doubt that people sleeping with eyes closed in urban environments would have suppression of MLT. Typical room illuminations

The “precautionary principle”

The implications of these findings are that perhaps the “precautionary principle” should be applied in lighting applications. Carlin wrote:

The precautionary principle is no longer just a theory. Writing in Scientific American, David Appell notes how the precautionary principle is increasingly finding its way into international agreements [4]. He noted for example, that while it is already `a matter of law in Germany and Sweden, the precautionary principle may soon guide the policy of all of

Better lighting practices should begin now

Until more research directly links exposure to LAN to increased rates of human cancers, it may be wise to consider preventive measures in the application of everyday lighting practices.

The best way to daily entrain (reset) our circadian clocks and have our bodies produce a healthy dose of vitamin D is to get 15 min of natural sunlight exposure each morning [44]. At night we should sleep in total darkness. But in modern industrialized societies we see people with poor sleep habits. There are a

Conclusion

Lighting only for visual acuity or to achieve decorative effects is no longer acceptable [58]. Lighting must also be friendly to the circadian clock. We must recognize the body's need to sleep in darkness which allows for the normal night time production of MLT. MLT is a long-evolved protective hormone that should be allowed full expression during early morning hours.

On the basis of the growing body of animal research and human epidemiological studies the hypothesis that exposure to LAN may in

Acknowledgements

I wish to acknowledge the Lighting Research Office (LRO), a service of the Electric Power Research Institute, Palo Alto, CA, for holding their meeting on Light and Human Health in Orlando, FL, November 3–5, 2002. Neither the LRO nor the Electric Power Research Institute expressly endorses the ideas and content of this paper.

The Expedition Inspiration Fund For Breast Cancer Research and the late Laura Steele Evans who inspired thousands of breast cancer survivors were motivators for this paper.

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