Lighting is complex. designers worry about task visibility and visual comfort, color rendering and flicker, and how lighting products and lighting patterns support the mood, safety, function, and appearance of spaces. Now there is a new consideration: Researchers are learning that the natural cycles of light and dark are important for maintaining human health. It is important for us to be exposed to bright light during the day, and equally important to experience darkness at night. There is growing evidence that exposure to white or bluish light at night negatively affects daily biological rhythms, sleep quality and the immune system.1 This is likely to impact how we light interior and exterior spaces in the future.
Light's effect on our circadian rhythms has been recognized for years, but only recently has it been better understood. In addition to rods and cones in the retina, the human eye has a set of retinal ganglion cells that receive and convert light signals into electrical signals, and then transmit them to the suprachiasmatic nucleus (SCN) in the central brain. The SCN is the body's timekeeper, and it in turn sends signals to hormone centers including the pineal, pituitary, and adrenal glands. These control the secretion and suppression of hormones, such as melatonin, seratonin and cortisol, and a host of neurotransmitters, such as acetycholine, dopamine, and norepinephrine.2 Bored, yet? What if I told you that without these essential chemicals circulating at precise cycles, you wouldn't be able to regulate wake time, hunger, and body temperature as consistently; you wouldn't sleep as well; you might be more susceptible to depression; damaged cells wouldn't get repaired properly; and your immune system wouldn't be able to fight disease as quickly?
The color of light that ganglion cells detect is different from the color detected by rods and cones. While your cone (daytime) vision is most sensitive to the yellow-green portion of the visible spectrum, and your rod (nighttime) vision is centered on the blue-green area, the blue-indigo portion triggers a greater response from these biological receptors. They are most affected by short wavelengths, in the range of 460 and 480 nanometers. This may correspond with the color of the blue sky under which humans evolved.
Exposure to bright light during the day resets your internal clock, and helps regulate sleep/wake cycles and fight depression. Work by Figueiro and Rea shows that exposure to bright light at night acts 'like a cup of coffee' to make night nurses more alert.3 The amount of light needed to produce this biological effect is under investigation. For example, doses of 250 footcandles measured vertically at the eye and administered for a couple of hours seems to help reset the biological clock, a treatment for sleep disorders, seasonal affective disorder and jet lag. (There may be a time-dose relationship, such that 1,000 footcandles of exposure for X hours may be equally effective, but this requires more research.) These are very high illuminances, and the best way to get them is to walk outside, where daylight levels can exceed 3,000 footcandles even under cloudy skies.
Light levels at night are a different story. Work by Dr. George Brainard of the Jefferson Medical College has shown that administering far lower light levels (as low as 0.1 footcandles for short, or blue, wavelengths) at night can suppress melatonin.4 Since melatonin seems to have important anti-oxidant effects, and may work in concert with other disease-fighting cells under the presence of darkness, light pouring into bedroom windows at night may cause long-term health consequences. Epidemiological evidence reported by Dr. Richard Stevens and others points to a possible link between exposure to light at night and increased rates of breast cancer and colorectal cancer.5
If research continues to produce such results, it means that humans need bright days and dark nights to maintain their health, with high doses of blue-rich light early in the day, subdued light in the evenings, and dark sleeping environments. If it is impossible to achieve truly dark nights, then low levels of red, orange or amber light are less disruptive than exposure to white or blue light.
Applying Research to Design
Given the direction this research is pointing, will offices, factories and schools be illuminated to 250-plus footcandles in the future? It's too early to know, but my guess is 'no.' Such light levels would be at least five times greater than those we target now; the energy used would be expensive, and power production would contribute significantly to air pollution. However, it might make sense to provide high light levels in common areas such as cafeterias, break rooms, and exercise rooms. Building occupants could take a break and simultaneously get their dose of 'healthy light.' Daylight would be used when available, and supplemented with electric light when unavailable, which would be provided by blue-rich sources like metal halide or fluorescent, or by blue LEDs with traditional sources. Because of where the biological receptors are located in the retina, smearing light on the ceiling and upper surfaces of a room would be more effective than putting it on the floor or desktop.
By far the most significant effect this research could have on buildings is daylight, daylight, and more daylight. Intelligent daylighting does not mean punched openings in walls. It means designing building floors with taller ceilings and window openings to help daylight penetrate spaces more deeply. Increasingly designers will site buildings according to solar angles, and incorporate façades with overhangs, awnings, light shelves, skylights, clerestories, and sawtooth ceilings to prevent glare. And they will specify glass with a spectral transmittance that admits the blue portion of the spectrum. (I wouldn't buy stock in companies that manufacture only bronze-tinted glass right now!)
Integration of electric lighting and daylighting will encourage the development of more fluorescent and metal halide dimming systems. Interior layouts and finishes may be affected, since the lighter the surface paint colors, the higher the light levels reflected at the eye. Workstation partitions absorb light, so partition heights will likely drop, especially near windows.
Buildings may also become narrower. In the 1900s, industrial and office buildings were narrow to allow daylight to penetrate the space. Electric lighting enabled a larger footprint, but this research may reinforce the importance of employee access to windows and that biologically essential daylight.
Buildings Most in Need
Think about buildings where occupants don't get outside much: Nursing homes, mental institutions, prisons, hospitals, or industrial plants. Even people in home offices or home-bound parents may not leave the building during the course of the day, and therefore may not get the blast of light they need. This is exacerbated at latitudes with short winter days. These spaces could use therapeutic doses of electric lighting during the day.
Our corresponding need for dark nights may also influence the design of buildings where people sleep-nursing homes, institutions, prisons, hospitals, school dormitories and military barracks. Research has not yet determined how much white light is too much, but it is a good idea to design bedrooms with window coverings that block outside light. If it is determined that restroom and corridor lighting should minimize blue wavelengths at night, we could see a separate system of orange lighting or LEDs that emit only long wavelength light in the nighttime hours. Smart switches and automatic panel control systems that disable white lighting at night may also become common.
Nightshift workers pose a particular challenge. Since light at night suppresses melatonin, it can help workers stay alert. However, this may also negatively affect their long-term health. We must wait for more conclusive research results. For now, blue-blocker sunglasses may help them if they walk or drive home in the daylight; this minimizes the blue wavelengths that seem to interfere with the ability to sleep.6 Researchers also stress that it is important for nightshift workers to sleep with dense eyeshades and/or in completely dark rooms.
Research into light and health is in its infancy. Our industry needs to come forward to provide financial support to learn more about this important area, developing application guidelines for real world spaces. Stay tuned. In the meantime, go outside for a half hour of exercise and light exposure. Not only will we get our light therapy, but we'll shed a few pounds, too.
Principal of Naomi Miller Lighting Design in Troy, New York, Naomi's interests include lighting quality, the aging eye, health effects of light, dark skies and energy efficiency. She is a member of the Lighting Research Office's Technical Advisory Committee.