The earliest light therapy devices were large and ungainly—basically fluorescent ceiling fixtures, like we still see in offices, which were modified with extra bulbs to sit on a table or counter. These light boxes provided doses of 2500 to 10,000 lux, with the lower levels requiring longer treatment sessions. Gradually the units were made smaller and less imposing, but they maintained a broad field of illumination at the eyes.
There are important limits to downsizing. To be effective, the field of illumination at the eyes has to be large enough to keep them in the therapeutic range. It is easier to glance away from a smaller light source. Easier … and more likely, too, because if the light source is too small it triggers aversive glare, which heightens the urge to squint or look away. As a result, miniaturization probably lowers the dose of therapeutic light, compared to the basic research studies. As the nonprofit Center for Environmental Therapeutics put it, “Smaller is not better: When using a compact light box, even small head movements will take the eyes out of the therapeutic range of the light.”
Another source of variation: From early in the 30-year history of the field, imaginative designers reasoned that if you could somehow attach lights to your forehead and beam them down at the eyes, you would have a therapy that allowed you to walk around, untethered to your desk or kitchen table. Bulbs were even attached to eyeglass frames. With the lights so close to the eyes, not much power would be needed, so they could be run on batteries.
One company even got funding for a controlled clinical trial, but the result showed no difference from dim, sub-therapeutic light. As one of the investigators concluded: “It is clinically useless.” In addition, some patients complained that it restricted their upward vision, caused scratchy eyes, or just felt uncomfortable. Especially with the spread of LED-based technology, we see the same kind of head-mounted devices show up again and again, commercialized without standard clinical testing for safety and efficacy.
We should also note that this mode of illumination is unique in human history. We evolved with the ability and need to continually move our head with respect to ambient light from the sky, which varies the retinal response dynamically. (The same is true of indoor lighting.) As sensory physiologists have known for a long time, unless a stimulus provides continual variation, the light/sound/smell/touch receptors will adapt to it and become insensitive. This leaves a potential safety risk, never assessed, for head-mounted lighting devices.
And what about colored light? White is so-o-o boring! Color therapies—even taking a bath in colored light!—have long been staples of the untested, commercialized, and widely debunked armamentarium of “alternative healing.” There was an early claim that wearing rose-colored glasses could relieve SAD. Others maintained that invisible ultraviolet light close to visible blue was especially beneficial, even though it is patently dangerous and can lead to tanning addiction. One company promoted green light, based on the hypothesis that the antidepressant effect is maximal when the retinal receptors for sensory effect—vision—are selectively stimulated. We have learned since, however, that antidepressant effect is not exclusive to green.
Not that color is irrelevant. Recent lab research has convincingly demonstrated that light at the short-wavelength end of the visual spectrum—corresponding to violet, indigo, and blue—does have a selective effect on the circadian timing system, especially with long exposures. This is one good reason to cut down on exposure to these wavelengths during the evening hours—a point we discuss in Chapter 5, “Getting Light Wrong,” of our book, Reset Your Inner Clock.
To Market, To Market We Go! Put a light therapy device on the market that features blue or bluish light, and you have a toutable difference from your competitors. Unfortunately for this business plan, blue light devices provide no clinical benefit over well-tested white light boxes. There are also concerns about safety and efficacy, as well as visual comfort. Irradiation of the retina with blue light can interact with several common medications, and there are cautions especially for people 50 and above who have, or who are at risk for, macular degeneration. Despite the contrary claims of lighting engineers, the continual use of blue light devices may have adverse biological consequences. These may take decades to build up, so they would not be evident with short-term testing.
Let’s say you pay attention to all these caveats and search out a light therapy device that has been thoroughly clinically tested. What now? How do you use it? A simple question, but the answer is not.
The primary dosing parameters of light are its field of illumination, intensity and spectral content (even within variations of white), and the length of exposure sessions. But the ultimate effect depends heavily on when you use the light within your own particular circadian cycle, as reflected in your chronotype. You could seek guidance from a doctor who is skilled in in light therapy—and there is a growing number of them. Short of that, we wrote our book to be a tool to help consumers and patients develop an effective, personalized, treatment strategy.
Michael and Ian are co-authors of the 2013 Penguin paperback, Reset Your Inner Clock. They invite you to follow them on Twitter for news updates, opinions, and challenging Q-and-A’s. If you want to stay on top of body-clock matters, light therapy, and more — and take advantage of confidential, online self-assessments of inner clock time, depression, and seasonality — you should become part of the nonprofit Center for Environmental Therapeutics community. Email PTuser@cet.org so we can stay in contact.