Blue-free white light breaks the paradigm of circadian lighting
Non-visual receptors in humans are especially sensitive to blue spectral energy, explains AURELIEN DAVID, and the latest LED technology enables high-quality white light with reduced blue content that can enhance circadian health.
The impact of light on our sleep cycle and health is a major medical discovery of the last two decades. Artificial light, which is ubiquitous in modern societies, can influence the circadian cycle just like daylight does. This is both an exciting opportunity for the solid-state lighting (SSL) sector, and a cause for potential concern. Let’s discuss what is known about the non-visual aspects of light and consider how LED-based sources can evolve to enhance human health and wellbeing.
In the early 2000s, based on seminal research on light and sleep, researchers identified yet-unknown receptors in our retinas: the so-called intrinsically-photosensitive retinal ganglion cells (ipRGCs). These receptors are sensitive to light, but rather than causing a visual stimulus (like the well-known rods and cones), they connect directly to the part of our brain that regulates our sleep patterns – the circadian cycle. For more background, see an LEDs Magazine interview that discusses an article from academics on the topic.
What is most important about these receptors is their sensitivity to blue light – their peak sensitivity is at a wavelength of about 450-480 nm. When excited with light, they send a wakeup signal. This signal is observed in various physiological responses; for instance, it prevents our bodies from releasing the hormone melatonin, which strongly correlates to our circadian cycle. This maximal sensitivity to blue light makes sense as it relates to natural sunlight: Blue radiation is more prevalent in the morning, diminishes throughout the day, and is absent at night; therefore, our bodies use it as a cue to synchronize the internal clock.
Opportunities and challenges
On the one hand, it is possible to provide additional light to help those who wouldn’t get enough from natural conditions. For instance, a jolt of blue light in the morning may help those living in sun-deprived locations, or populations whose circadian cycles are naturally less well-regulated such as teenagers and the elderly. We are especially sensitive to early-morning blue light, in the first one to two hours of our day.
On the other hand, exposure to light in the evening can have unwanted effects. Research has shown that common indoor lighting conditions (100 lx and above) are enough to significantly impact the circadian cycle. Such effects persist for several hours after light is turned off, and can thus delay the onset of sleep. Display light from phones and tablets is an additional cause for concern. This worry has become even more acute with the spread of LED lighting, which uses blue-pump LEDs whose intense blue radiation can be especially impactful.
The topic has become prominent enough to trigger statements from a variety of authorities, such as the American Medical Association (AMA) and the US Department of Energy (DOE). LEDs Magazine, for instance, has published a number of articles focused on recent AMA recommendations for warm-CCT outdoor lighting. Influential tech companies, as well as big players in the lighting industry, are attempting to address blue-light concerns in their most recent circadian-friendly products, including displays and light sources. But what is the underlying technology, and what are its limitations?
Tuning spectra for the circadian cycle
Circadian entrainment is proportional to the total dose of blue light, which is the product of two factors: the total light amount where more light causes higher stimulation, and the relative amount of blue radiation in the spectrum of the light where more blue radiation causes higher stimulation. To influence the circadian cycle, one may therefore alter the light level and the spectrum.
Various metrics have been proposed to quantify the total dose of blue light, such as the melanopic lux defined by Lucas. Their accuracy is still being assessed – a task that is complicated by our imperfect knowledge of the circadian action spectrum. However, it is well-accepted that this action spectrum has a peak sensitivity at about 450-480 nm. Therefore, modifying the spectrum of light in this range provides the most opportunity to affect circadian entrainment.
LED technology makes it possible to shape a spectrum as desired, and in particular to reorganize the spectrum to add or remove blue radiation, but this must be done while maintaining other important properties of the light. A crucial aspect is the chromaticity of the light itself, which should remain white. This constraint is not trivial: If one simply starts from a white light source and removes all blue radiation (for instance, with a blue-suppressing filter), the resulting light suffers from a very yellow-greenish tint that is too unpleasant for practical use. Therefore, decreasing the circadian stimulation of a light source can’t be reduced to simply removing blue radiation.
Morning and evening needs
White light is characterized by its correlated color temperature (CCT). High-temperature light – for instance, daylight – contains more short-wavelength radiation, whereas low-temperature light is richer in red radiation (Fig. 1). This leads to a simple strategy to tune the circadian stimulation of a light source by tuning its CCT and intensity – higher in the morning and lower at night. The human vision system naturally adapts to light of different CCTs through the process of chromatic adaptation, so a wide range of CCTs is acceptable for general lighting.
This approach works well to increase circadian entrainment in the morning, by raising the CCT to 4000-5000K and increasing light intensity to 1000 lx and beyond. Various medical studies have confirmed that blue-rich, high-CCT light could help synchronize the sleep schedule, in particular for populations whose circadian cycle is otherwise altered by a medical condition. Accordingly, the lighting industry has embraced this concept and introduced a number of products with enriched blue radiation in the morning.
On the other hand, it is much more challenging to create a source with low circadian stimulation in the evening. To reduce the total blue dose, both the amount of light and the spectrum must be adapted. Light intensity can be reduced somewhat, say from a typical 100-300 lx down to 30-50 lx, but lower values become too dim for domestic use. An even more serious challenge arises when trying to obtain a spectrum with very little blue radiation, as lowering the color temperature reaches a limit. If the CCT goes far below 2700K – the temperature of an incandescent bulb – chromatic adaptation becomes imperfect. The light appears yellowish despite being technically characterized as white light since it lies on the black-body locus. An example is the light produced by a candle; with a CCT of 2000K, it may be pleasant in specific contexts but looks far too yellow for most domestic activities.
FIG. 1. The graph depicts chromaticities of various light sources. The black line shows the black-body locus of white light (sources far away from this line don’t appear white). Removing blue radiation from an incandescent light makes it yellow-greenish. Candlelight, although technically classified as white light, has a pronounced yellow tint.
Today, all commercial sleep-friendly bulbs use this low-temperature approach (with temperatures in the range from 2000-2300K) and thus suffer from an unnatural light color. Because of this problematic color quality, manufacturers suggest that such bulbs only be used in a few lighting fixtures, for instance, bedside lamps. It is worth noting that the effectiveness of these products so far hasn’t been confirmed by clinical studies.
This same approach is also used for displays: From computer utilities like f.lux to smartphone features such as Apple’s Nightshift, the amount of blue radiation is reduced by lowering the color temperature of the screen at night. Here again, the temperature can only be lowered to a certain point (about 3500K) before the screen looks unacceptably yellowish.