How Light Affects Your Health, Day and Night

As days lengthen through spring my thoughts often turn to… light. Every year I find my spirits lift as the days get longer and I can spend more time outdoors. I see this as yet another example of how much better we feel when we’re living a lifestyle that’s close to “natural.” Curious to see what the latest science shows, I looked for studies on how we can make the most of this seasonal bounty — not only during these bright and sunny days, but at other times as well.


Recognizing the many health benefits of natural light, the Department of Architecture at the Massachusetts Institute of Technology is studying how to make the most of natural daylight and use less artificial light in buildings. They are developing tools to manipulate illumination to meet human circadian needs so architects can make healthy light-related design decisions.

One recent study, published in the Scandinavian Journal of Work, Environment & Health, found that using white light enriched with blue-light waves helped office workers stay more alert during the day, reduced evening fatigue and improved night sleep. Investigations are also underway to see how other types of light treatment — such as synchronization to individual melatonin rhythms, simulated dawns or adding environmental blue light — might improve individual health and performance.

To learn more about these new findings, I spoke with neuro-endocrinologist, Russel J. Reiter, PhD, at the University of Texas Health Science Center at San Antonio, a long-time researcher on this topic. I asked whether it makes sense to purchase different types of lights for different rooms in our homes, based on these findings. Affirming the connection between blue light and alertness, he told me that white lights (which contain all wavelengths, including blue) are best for rooms where you are reading, working or engaging in other activities that require focus. In rooms where you want to relax, such as the bedroom, it makes sense to use bulbs that have a yellow-to-red cast, which do not have this effect.


The darker side of this story is that there is evidence that the wrong light can actually damage your health. I spoke with Richard G. Stevens, PhD, a cancer epidemiologist at the University of Connecticut, who has been studying the rise in breast cancer rates that occurred along with industrialization. He told me that increasing use of artificial lighting during the nighttime hours might be disrupting workers’ circadian rhythms, possibly by suppressing melatonin production. Melatonin is produced by the body at night and is known to inhibit tumor formation — however, experts theorize that when the retina encounters light at night, production shuts down and cancer risk increases.

Researchers at the University of Haifa in Israel used satellite images to measure light at night in a variety of communities, finding that those with the highest levels also have the highest breast and prostate cancer rates. To check this theory, a comparison group — blind women unable to see light at night — was measured in four countries and, indeed, found to have 20% to 40% less cancer risk. Dr. Stevens told me that in 2007, after careful study, the International Agency for Research on Cancer identified overnight shift work as a probable carcinogen and he predicts that we will begin seeing more studies focused specifically on the cancer risks of “light at night.”


Then, of course, is the increasingly well-known link between sunlight, vitamin D production and overall health. For the latest details, I called Michael Holick, MD, PhD, director of the General Clinical Research Center at Boston University School of Medicine.

Vitamin D is unique, he says, “because we make it in skin exposed to sunlight.” It helps absorb calcium for bones, supports immune systems and reduces common cancers, cardiovascular disease, high blood pressure, types 1 and 2 diabetes, rheumatoid arthritis, multiple sclerosis and infections.

The sun has been the natural source for vitamin D for hundreds of thousands of years, Dr. Holick noted. Vitamin D-deficiency rickets became a health problem when industrialization began to keep people working indoors all day. He believes more than half of Americans are vitamin D-deficient today because they avoid sun altogether, overuse sunscreen and spend too much time indoors, in front of computer screens and in artificial light. Dr. Holick offered some general recommendations for the spring and summer months: Get 15 minutes of arm and leg exposure two or three times a week, between 10 am and 3 pm. (Note: This is the guideline for those whose skin turns pinkish in a half hour of sun.) Sun exposure requirements vary by latitude, time of day and skin pigmentation, so adjust accordingly. Use sunscreen with at least an SPF 15 on your face, since it is more prone to skin cancer, but on your body only after your initial exposure, because it significantly reduces your vitamin D-making ability. This should generate about 1,000 IU of vitamin D per day, which Dr. Holick says is enough.


And, of course, light — or rather the lack thereof — has an effect on how well we sleep at night. Here are some tips from our experts…

  • Darken your room as much as possible. Turn off your computer monitor. Don’t sleep with the television on. Don’t use a nightlight in your bedroom.
  • If you need light for safety — for instance, in the hallway or bathroom — use low-wattage and preferably red light bulbs in a nightlight. (These are available in hardware stores — Dr. Stevens suggests using Christmas bulbs if you can’t find other red bulbs.)
  • Use light-blocking shades to keep outdoor lights from brightening your room. If “trespass light” leaks in anyway, wear eyeshades while sleeping — it’s not dangerous but you can minimize its effect on your circadian rhythm by blocking it out.

If you wake up and want to go back to sleep, do not get up and go into a lighted room. Instead, lie quietly in the dark — counting sheep, your blessings or whatever works for you — while you wait for sleep to return.

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