Blue Light is Not More Effective than White Light
for Light Therapy or Human Circadian Regulation
The Controversy Regarding the Benefit Of Blue Light for Light Therapy
The concept that blue light wavelengths are beneficial for light light therapy and circadian phase regulation became popular with the publication of a paper by a group of Harvard Sleep Medicine researchers (and G.C. Brainard) in 2006 in the journal Sleep that claimed blue light wavelengths were the major factor in the regulation of human chronobiology by light. See Short-Wavelength sensitivity for the Direct Effects of Light on Alertness, Vigilance, and the Waking Electroencephalogram in Humans. S.W. Lockley et al. This Harvard paper became a highly cited source in the field of chronobiology leading to the widespread acceptance among both the public and chronobiology researchers working on human sensitivity to visible light, and supported the incorrect perception that photosensitive melanopsin in iPGCs (intrinsic Photoreceptive retinal Ganglion Cells) was primarily responsible for light regulation of human circadian physiology.
In spite of the editorial by S.S. Campbell in that 2006 issue of Sleep questioning several comments and conclusions of the Harvard 2006 Sleep paper by Lockley et al., the Lockley paper has led to major misunderstandings regarding spectral sensitivity of light therapy and the circadian phase regulation of humans by light that are now common.
Based on these findings, and on the tenor and focus of the authors’ discussion, it would be easy to come away from this paper with the distinct impression that light-induced improvements in alertness and performance are mediated almost exclusively by wavelengths in the 460nm range. This conclusion would be inaccurate, on several counts. First, despite the authors’ repeated use of the term, it appears that neither performance nor alertness were actually “improved” by exposure to either light source. Rather, the typical circadian dip in those measures appears to have been attenuated more so by exposure to 460nm than by exposure to 555nm. Yet, comparisons to other wavelengths, combinations thereof, a broad-spectrum, 460nm“knockout” light source, or some other appropriate control were not made. As such, the only conclusion that can be drawn from this study is that we are more sensitive to monochromatic blue light than we are to monochromatic yellow light[ our emphasis]. Moreover, because the study did not include a control condition, and because the authors did not analyze each condition relative to a circadian phase-equivalent baseline, it is impossible to assess the degree to which alertness and performance were actually affected by exposure to either wavelength."
"Another methodological problem involves the fact that timing of light exposure was scheduled to occur during an interval
that can also result in significant circadian phase delays (6.5 hours, ending 15 minutes prior to the presumed nadir of body temperature).
It is impossible, therefore, to effectively differentiate between possible circadian effects on alertness and performance and those attributed
by the authors solely to acute activating effects. This confound is particularly problematic when trying to interpret the finding that
subjective sleepiness remained relatively low for up to an hour after exposure to the short-wavelength light was terminated. While the
authors conclude that this likely reflects a carry-over effect of the direct activating effects of light, the alternative interpretation
that it was the result of a differential phase delay induced by exposure to 460nm cannot be ruled out."
See full Campbell editorial: Short-Wavelength Sensitivity for Activating Effects of Light:
An Ascent to the Arcane?
Harvard Concedes Lack of Benefit from Blue Light for Light Therapy
In 2010, the Harvard university group, (with G.C. Brainard), published a follow up study in a Science journal, Science Translational Medicine, Spectral responses of the human circadian system depend on the irradiance and duration of exposure to light, addressing the concern raised in the 2006 Sleep editorial regarding the 6.5 hour duration of light exposure used in their 2006 study. This paper acknowledged that increasing the proportion of blue light wavelengths in light therapy has no benefit, and does not improve the effectiveness or efficiency of light therapy.
"Our results indicate that short-duration (<90 min) retinal exposure to narrow-bandwidth 555-nm [yellow-green] light may be as effective, if not more effective, than an equivalent photon dose of 460-nm [blue] light."
"Our findings have implications for the development and optimization of light therapies for a number of disorders, including circadian rhythm sleep disorders, seasonal affective disorder (SAD), and dementia, and the use of light as an alerting stimulus to counter the sleepiness associated with misalignment of circadian phase, particularly during night shift work."...Doing so holds the promise of reducing light therapy duration and intensity, thus possibly improving patient compliance and safety."
"Finally, blocking short-wavelength light with blue-blocking goggles may not always be effective in preventing undesired circadian responses based on our finding that longer-wavelength light is able to induce robust phase-shift responses."
Spectral responses of the human circadian system depend on the irradiance and duration of exposure to light.
J. J. Gooley, S. M. W. Rajaratnam, G. C. Brainard, R. E. Kronauer, C. A. Czeisler, S. W. Lockley; Sci. Transl. Med. 2, 31ra33 (2010).
Since light therapy is generally used for less than 90 minutes, and since green light with wavelengths around 555nm have been shown to be no more effective for light therapy than white light, there is no demonstrable benefit from increasing the proportion of blue light wavelengths in a light source used for light therapy. One of the authors, GC Brainard, had even published a study in 2001 showing that 505 nm green light had a much stronger effect on melatonin suppression than 555 nm green light. REF
Another issue raised in this paper was regarding the age related yellowing of the human lens. It was noted that the subjects used in this study were under 30 years of age, which is meaningful because studies have shown there is a substantial decrease in the efficacy of blue light therapy in people over 40 years old. This results from the age-related yellowing of the lens which increasingly and substantially limits the amount of blue light reaching the retina after age 40.
The 2006 Harvard SLEEP paper had been considered particularly significant in the study of human chronobiology due to the credibility of the Harvard
Sleep Medicine Department under CA Czeisler, who published one of the original papers on this subject, and the extent of their research on the use of light therapy to
regulate human circadian phase. Although several studies have previously found that blue light wavelengths are not particularly effective for light therapy, this study is
particularly significant because it was was widely cited to justify the use of blue light to improve the efficiency of light therapy lamps and it appeared to establish the
importance of blue light for light therapy by indicating that human physiology is most sensitive to exposure to blue light wavelengths; a misunderstanding that persists until
today among several circadian researchers as well as the public.
This misunderstanding was promoted by several of the authors of the 2006 Sleep paper. For example in 2008 Lockley made a presentation to the Aerospace Medical Association (AsMA)
where he stated:
"A wavelength-dependent shift in the sensitivity of the dose-response curves showed that 460 nm light was at least twice as effective as 555 nm light for circadian phase resetting and melatonin suppression (range, ~1.0-3.0 x 1013 photons/cm2/s). The short wavelength-sensitivity of human circadian responses to light is consistent with a role for the blue light-sensitive photopigment melanopsin in circadian phototransduction. These findings have important implications for light therapy-based treatments for sleep and circadian rhythm disorders associated with shiftwork, insomnia, and jet-lag."
AsMA presentation- Lockley et al
The comment, Light Matters, on the cover of the 2010 Science Translational Medicine issue this paper was published in, proposed: "blue light now often used for therapy in depression or shift work should perhaps be replaced by green or white illumination.". REF
Safe Light Wavelengths provided by Lo-LIGHT Lamps are Most Effective.
In 2022 a group of light therapy researchers in the Sleep Medicine Department at Harvard (plus G.C. Brainard), published a paper in PNAS (Proceedings of the National Academy of Sciences) on the effects of several different wavelengths in the visible light spectrum on human circadian physiology, which indicated that the light wavelengths provided by Sunnex Biotechnologies Lo-LIGHT lamps, i.e. those in the 500 - 510 nm portion of the visible green light spectrum, are the most effective light wavelengths for light therapy. The claimed "Examination of the fitted half-maximum value of each irradiance response curve shows that 507 nm light is more effective than 480 nm light at suppressing melatonin and resetting circadian phase"
It appears that this 2022 PNAS paper was a further response to the 2006 Sleep editorial that was quite critical of the Harvard 2006 Sleep study. The initial response to the editorial by SS Campbell was the 2010 Harvard paper in Science Transational Medicine that addressed the criticism of the 6 1/2 length of light exposure in that study. The 2022 PNAS paper addresses a second main criticism, the comparison of only 2 wavelengths, 460 nm blue light with 555 nm yellow-green light. The 2022 PNAS paper examined the effects of several wavelengths across the visible light spectrum. One reason the PNAS paper appears to be a response to the critical 2006 Sleep editorial is because the study described in the 2022 PNAS paper was conducted prior to 2010, even though it was not published until 2022.
"We constructed radiance-response curves and action spectra for melatonin suppression and circadian resetting responses in participants exposed to 6.5-h monochromatic 420, 460, 480, 507, 555, or 620 nm light exposures initiated near the onset of nocturnal melatonin secretion."
"Healthy research subjects (n =122), ages 18 to 30 y were enrolled in a 9-d inpatient study at the Intensive Physiologic Monitoring Unit (IPM) in the Center for Clinical Investigation (CCI) at Brigham and Women’s Hospital (BWH; Boston, MA) between 2000 and 2009."
PNAS study, 2022
In 2024, Dr. C.A. Czeisler, the head of Harvard's Department of Sleep Medicine, along with several longtime researchers of light therapy at Harvard, acknowledged in the journal Sleep Health that for the initial 90 minutes of light exposure, the human physiological response to light, as measured by either melatonin suppression or by its capacity to shift circadian rhythms, is not in the blue region of the visible light spectrum, as is widely presented in the light therapy literature, but is in the green portion of the spectrum, in the region of the spectrum emitted by Lo-LIGHT lamps - i.e. 500-510 nm.
"given the effectiveness of monochromatic blue-green light (507 nm) at both suppressing melatonin secretion and delay resetting circadian phase and our earlier finding that dim monochromatic 555 nm green light may be more effective than dim blue light at inducing circadian phase resetting, these findings with green-enriched polychromatic light challenge the oversimplification that blue light is always more effective than green light in inducing circadian resetting responses in humans."
"for subjects whose eyes are not dilated, and who have not been restricted to very dim light exposure for about a day prior to the therapy, and who use light therapy for less than 90 minutes per session."
"In the GREEN light conditions (GG and GS) the ambient room light was generated by ceiling-mounted green, fluorescent lamps (Sunnex Biotechnologies, Winnipeg, MB Canada)"
Harvard's NASA study with Lo-LIGHT lamps
Again its worth noting that while the results of their study with Lo-LIGHT lamps was published in Sleep Medicine in 2024, The Harvard NASA study with Lo-LIGHT lamps from Sunnex Biotechnologies was conducted in 2009-2012.
"We predict that, in contrast to white light, simple exposure to polychromatic green light throughout the day will rapidly (within five days) entrain the circadian melatonin rhythm to the shifted sleep-wake schedule, without the need for bright light exposure-rendering obsolete the crew quarters' bright light facility and enabling implementation of this new technology to ensure circadian synchronization both during the pre-flight quarantine period and while aboard NASA flight vehicles""
"new information about the peak sensitivity of the human circadian system, determining that the most efficacious specialized light source should have a peak near 500nm (as opposed to ~470nm as the lamps produced by Philips Lighting, Eindhoven, Netherlands, as originally proposed), and therefore identifying such a new specialized light source for our studies; retrofitting our laboratory with specialized lamps (manufactured by Sunnex Biotechnologies, Winnipeg, MB, Canada) with a peak sensitivity near 500nm." NASA Taskbook - Evaluation of Photic Countermeasures for Circadian Entrainment of Neurobehavioral Performance and Sleep-Wake Regulation Before and During Spaceflight"
"Research Impact/Earth Benefits: We implemented and tested a new polychromatic fluorescent lamp with a peak spectral sensitivity of ~500 nm. This is near the peak sensitivity of the human circadian system, and thus should be the most efficacious polychromatic lamp for shifting the timing of the human biological clock. In addition to benefits for NASA flight personnel this technology will also have application to shift-workers, to jet travelers, and to any personnel who need to shift the timing of their biological rhythms." Final Report: Evaluation of Photic Countermeasures for Circadian Entrainment of Neurobehavioral Performance and Sleep-Wake Regulation Before and During Spaceflight. Last Updated: 01/08/2013
The paper published by the Harvard (plus Brainard) group in 2010 in Science Translational Medicine
indicated that in 2009 the authors were likely aware that for the first 90 minutes of exposure, blue light wavelengths are
not particularly effective for light therapy. Since light therapy sessions usually last less than 60 minutes, often closer
to 30 minutes, blue light does not contribute to the effectiveness of light therapy. Though this 2010 paper only examined
the blue wavelengths near 460 nm and the yellow/green wavelengths near 550 and was published much earlier than the 2022 PNAS
paper that examined the relative response to a range of light wavelengths referred to above, the study described in the 2022
paper was conducted prior to the publication of the 2010 Sleep study and had by many of the same authors as the 2010 study.
The 2010 paper by Harvard University's Department of Sleep Medicine demonstrated that increasing the level of blue light
wavelengths does not improve the effectiveness of a light therapy lamp to influence human physiological functioning within
the time frames that light therapy is commonly used.
See Link
Experts in the spectral sensitivity of human circadian physiology have recommended that all light therapy devices screen out potentially hazardous blue light wavelengths, and that light therapy devices emitting blue wavelengths of light should not be used. As one expert in light therapy stated,
"It should be noted that broad-spectrum white light, traditionally used for bright light therapy, also contains blue light of potential concern particularly for very high intensity, long-duration exposure. Clearly, the safety of bright light therapy for people needs investigating. In the meantime it would be suggested that light in the 500 to 530 nm wavelength range (blue-green) should still be effective while avoiding the putative blue hazard". REF
Several studies using low intensity Lo-LIGHT lamps, which emit no blue light and cannot damage the eye, have been published in highly rated journals by leading authorities of light therapy. These studies demonstrate the effectiveness of low intensity GreenLIGHT technology in regulating human circadian rhythms and treating mood disorders. One shows that Lo-LIGHT lamps are least as effective at inducing physiological responses than a blue-enhanced (465 nm) light therapy device that emits 10 times as much light. see ref3.
Blue light and light therapy.
Although some earlier studies indicated that blue light would be highly effective for light therapy, it has now been determined that hazardous blue light wavelengths do not contribute to the effectiveness of light therapy. Studies have shown that monochromatic blue (479 nm) light is no more effective for light therapy than regular fluorescent white (polychromatic) light. see REF These studies were unable to demonstrate any benefit or increase in efficiency from increasing the proportion of blue light wavelengths, i.e. visible light wavelengths shorter than 480 nm, from a light therapy lamp. It has been known for many years that the inclusion of blue light is not necessary for effective light therapy.
The theoretical understanding used to justify blue light therapy, that the absorption spectrum of melanopsin in intrinsically photosensitive ganglion cells in the mammalian retina would determine the spectral sensitivity light therapy, was ill-conceived.
As Altimus et al found:
"At low light intensity, ipRGCs lack sensitivity, whereas rods are known to respond to increasing light levels and thus reliably relay this information to higher centers. Rods will continue to signal persistent light exposure through the rod-cone pathway even under conditions where their photocurrent is saturated. Finally, at high light intensities and for prolonged light exposures, melanopsin phototransduction in ipRGCs will extend the range of light intensities that allow circadian photoentrainment."
Rod Photoreceptors Drive Circadian Photoentrainment across a Wide Range of Light Intensities. Nature Neuroscience 13, pages 1107–1112;
Altimus, Hatar et al
Lo-LIGHT Most Efficient LIGHT Therapy
It is now apparent that the wavelength sensitivity of human physiology to light exposure does not simply correspond to the spectral excitation sensitivity of melanopsin. These finding support Sunnex Biotechnologies earlier studies on the spectral sensitivity of the non-visual light response in humans, and help explain the effectiveness of the patented low intensity GreenLIGHT technology used in Lo-LIGHT lamps.
For research demonstrating the superiority of GreenLIGHT to blue light for light therapy See Here
The risk to vision from blue light therapy
Since blue light wavelengths do not contribute to the effectiveness of light therapy, and blue light promotes the development of Age-related Macular Degeneration (AMD), the use of blue or blue-enhanced light therapy lamps can only increase the risk of retinal damage and vision loss without providing any benefit.
Unlike bright light or blue light therapy lamps, the Lo-LIGHT therapy lamp poses no risk to the user's vision.
While the green light wavelengths near 555 nm used in the Harvard study referenced above are not very effective for light therapy, Lo-LIGHT lamps emit a narrow range of green light peaking in the visible light spectral region of 500 to 505 nm, which researchers at Harvard confirm is the most sensitive region of the spectrum for regulating human circadian rhythms.
The finding that blue light wavelengths do not increase the effectiveness of light therapy, contrast with studies using Lo-LIGHT lamps for the treatment of depression, regulating circadian phase, and on light induced melatonin suppression, which show that GreenLIGHT from a Lo-LIGHT therapy lamp is as effective as "bright" white light therapy that provides more than 20 times the intensity or brightness.
Link to light therapy and retinal damage
