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. 2010 Jan;118(1):A22-7.
doi: 10.1289/ehp.118-a22.

What's in a color? The unique human health effect of blue light

David C Holzman

What's in a color? The unique human health effect of blue light

David C Holzman. Environ Health Perspect. 2010 Jan.
No abstract available

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The 1998 discovery of a new photoreceptor in the eye—which later turned out to be especially sensitive to blue light—revolutionized the way we think about how circadian rhythm is entrained. Today we understand that blue light has many unique physiologic effects.
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Light: An Influential Environmental Agent Light acts on the body by two pathways: the primary optic tract governs visual perception and responses whereas the retinohypothalamic tract governs circadian, endocrine, and neurobehavioral functions. The retinohypothalamic tract is most sensitive to blue light stimulation—energy in the wavelength of roughly 459–485 nm. Source: Benjamin Warfield and George Brainard/Thomas Jefferson University. Adapted by Matthew Ray/EHP.
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Blue light, subtle and dramatic, surrounds us, its special properties serving many purposes. When it comes to light perception, glare and brightness are both functions of wavelength; the short wavelength of blue light appears relatively bright to human eyes, making this among the most energy-efficient colors of light to produce. The bright bluish light emitted by high-intensity discharge headlamps thus increases visibility while using less energy than halogen headlamps, but that brightness also can heighten glare for oncoming drivers, particularly elderly drivers, who may already have trouble seeing at night. Now-ubiquitous compact fluorescent lamps (CFLs) similarly produce more light with less energy compared with incandescent lamps, and the bluer the CFL (“daylight” bulbs have the bluest color balance), the more energy efficient. More dramatic blue light is found in dental offices, where blue curing lights are used to harden amalgam material (orange goggles and filters provide eye protection against the intense light). The specific wavelength and intensity of the curing light stimulates a photoinitiator in the amalgam to decompose and initiate polymerization of the compound. But don’t think blue light is all work and no play—some, like that cast by the sea of Christmas lights at Tokyo Midtown, serves little purpose other than sheer enjoyment.
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Blue light–emitting goggles, panels, and other devices are used to treat problems such as sleep disorders, jet lag, seasonal affective disorder, and premenstrual syndrome. But blue light doesn’t work solely through ocular stimulation; the shorter wavelengths can penetrate skin—this is how blue light is used to treat neonatal jaundice, in which the infant’s liver is unable to clear the normal hemolysis by-product bilirubin. Bilirubin builds up in the blood and enters body tissues, making the eyes and skin appear yellow. Blue light penetrates the skin and converts bilirubin into forms that can dissolve into the blood and be excreted in urine. The process repeats as untreated bilirubin continues to deposit into tissues from the blood, until most or all the bilirubin is converted.
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Why blue? Blue’s power to reset circadian rhythms is not intrinsic to the color. A photoreceptor for any color could have evolved to signal daylight to the suprachiasmatic nucleus. But the blues more easily penetrate the surface of the oceans—where life (and photoreceptors) likely first evolved—than do other visible wavelengths. The color balance of the sky may have helped to preserve blue’s clock-setting role throughout evolutionary history.
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Why blue? Blue’s power to reset circadian rhythms is not intrinsic to the color. A photoreceptor for any color could have evolved to signal daylight to the suprachiasmatic nucleus. But the blues more easily penetrate the surface of the oceans—where life (and photoreceptors) likely first evolved—than do other visible wavelengths. The color balance of the sky may have helped to preserve blue’s clock-setting role throughout evolutionary history.
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