The DOE has recently published a fact sheet titled Lighting for Health: LEDs in the New Age of Illumination. It summarizes the Trends in Neuroscience January 2014 article Measuring and Using Light in the Melanopsin Age. Both publications explain the current understanding of our visual and nonvisual response to light.
The basics of our visual response to light is understood by everyone – it gives us the ability to see. The nonvisual response is less known generally, and is still being researched by scientists across the globe. This is discussed in my book in Chapter 16 Light and Health. What we have learned in the past two decades is that there is a third type of light sensitive cell in our eyes (the first two being rods and cones) called the intrinsically photosensitive retinal ganglion cell (ipRGC). When light strikes the ipRGC a pigment called melanopsin breaks down, sending a signal to the brain. That signal doesn’t go to the visual cortex, however, but to the suprachiasmatic nucleus (SCN) the body’s timekeeper. The SCN regulates circadian rhythms and the production of hormones affecting alertness, heart rate, blood pressure, stress response, and more. The SCN is reset by information from the ipRGCs. Simple exposure to light, though, is not enough. The exposure time of day, duration, and wavelengths all contribute to proper synchronization. SCN regulation seems to be maintained by high brightness, short wavelength light in the morning (i.e., morning daylight). If appropriate stimulation does not occur, the timing signals for hormone production can become desynchronized. It is known that circadian desynchronization plays a roll in insomnia, mood, depression, reaction time, creativity, and alertness. It is suspected that this desynchronization also plays a roll in cancer, diabetes, dementia, and cardiovascular disease.
This has lead to some talk of light as a drug that controls the SCN. At this point it is probably premature to attempt to apply this information in most lighting designs because most spaces have a wide range of users with a similarly wide range of needs. A lighting design for the overnight shift, for example, may not work well for the day shift. There are a few rules of thumb that can be applied in specific circumstances. For example, a designer can minimize the nonvisual circadian response by limiting the amount of light, especially short wavelength light, reaching the eye. However, the science is still in the early days and the specifics about the effect of light level, spectral distribution, and timing on users and for various applications are not clear.