Seven years ago, researchers investigating the deleterious effects of artificial lighting on people proposed a minimum light threshold for melatonin suppression. Now they have performed experiments validating that minimum.
To cut short the suspense, the threshold proposed by researchers Mariana Figueiro and Mark Rea, from the Lighting Research Center at Rensselaer Polytechnic Institute, is exposure to 30 lux of white light at the cornea for 30 minutes, during the nighttime hours. They proposed in 2006 that above this level of exposure, some suppression of melatonin production would occur in most people. The research they just published in the open-access Journal of Carcinogenesis & Mutagenesis lends support, at least in a coarse-grained way, for a figure such as they had put forward.
Rea and Figueiro are not concerned specifically with LED lighting, but rather with any form of artificial lighting at night from incandescent on forward in the history of lighting. (These researchers use the acronym LAN for such lighting; others use ALAN.) It is known that nighttime exposure to light in the blue part of the spectrum is the main culprit in suppressing melatonin in humans. While LED lighting may exhibit a stronger blue spectral component than some other forms of lighting, all artificial light has some blue in it.
What the researchers wanted to know is where the cutoff lies for exposure to ordinary, "architectural," white artificial light. They chose to use light with a CCT under 2,700K for their studies. In particular, they outfitted goggles with two pairs of Philips Lumileds Luxeon M3-PW71 LEDs, one pair per eye. Those LEDs have a CCT of 2,670K ± 29K.
The researchers exposed a total of 28 subjects to corneal illuminance levels of 8, 22, 60, 200, or 720 lux for one hour. (The maximum blue light hazard-weighted radiance was calculated to be 1 percent of the safety threshold level for blue-light hazard.)
Melatonin levels were tested from blood draws performed before and after the light exposure. Sleep/wake diaries were compared with the readings from an Actiwatch-64 each participant wore for five weeks, which included the four weeks of the study.
The results are summarized in the graph. Light intensity is on the (log-scale) horizontal axis and degree of melatonin suppression on the vertical. The smooth curve is not a fit to the data points; rather it is the plot of the degree of suppression the researchers calculated from a model of the spectral sensitivity of the human circadian system. Note the relatively large size of the error bars (5 percent confidence level) especially for the lower levels of illuminance. No melatonin suppression could confidently be observed from 8, 22, or 60 lux at the subjects' corneas. Melatonin was suppressed by 19 percent at 200 lux and by 37 percent at 720 lux.
The researchers conclude:
A corneal light exposure of 30 lux for 30 minutes from "white" light sources used in most architectural lighting applications is proposed as a conservative hypothesized working threshold for nocturnal melatonin suppression.
— Keith Dawson , Editor-in-Chief, All LED Lighting