Blue light, filters and the circadian rhythm

Dr TC Botha

June 2022

Colour spectrum and retinal colour physiology

Visible light has a wavelength of 380nm to 740nm.^1,2

Colours are on ranges of wavelengths rather than a specific wavelength.¹

There are three cone receptors within the human retina to perceive colour.¹

Figure 3 above shows the different levels of stimulation of the individual cone receptors to create a spectrum of visible colour. The human eye, together with processing done by the human brain, can detect about 10 million different colours.¹

Human eye transmission of light and retinal damage: blue wavelength.

Only ultraviolet and blue light wavelengths are considered when discussing photochemical damage to the retina; for example, the threshold for retinal damage is 20 times higher at a wavelength of 533nm versus 440nm.^3,4

The cornea, along with our atmosphere, block all light of wavelengths shorter than 280-320nm.⁵ The lens blocks the UV spectrum up to 400nm.⁶ The wavelengths from 400nm to 500nm is transmitted in varying degrees dependent on age.^6–8 One study showed that for a child’s lens, the curve of transmission starts to increase sharply at about 390nm wavelength and has a 90% transmission at about 450nm.⁷ This same study showed that for an aged adult of 63 years of age, the lens’ transmission starts at about 400nm wavelength and only reaches 90% transmission at a wavelength of 540nm. It needs to be noted that there were only 9 human lenses measured, and the study was done in 1962. The study is often quoted as it was the first study of its kind to measure the wavelength transmission of the human eye. It also showed that in the very young age group, there is an 8% transmission window at about 320nm in the lenses less than 2 years of age. This is still a point of contention. This singular fact is used to argue that young children should not be exposed to any form of LED screen before the age of 2 years.^9,10 Few other studies exist to clarify the issue for us, the largest of which that remain unpublished by Barker et al in 1991.⁶ It does not fall within the scope of this review to argue this fact, but, to quote the American Academy of Ophthalmology: “there is no evidence that screen use harms children’s eyes”.¹⁰ It should be noted that this article is referring to the child’s physical eyes, not to their social development, general health, association of screen use with obesity, neurological development and depression, etc.

There is still conflicting evidence on blue light exposure and the degenerative retinal condition called age related macular degeneration.^11,12 It has been shown that evidence is lacking that sunlight exposure over extended periods can cause this disease either.¹³ Blue blocking intraocular lenses have been designed to mimic an older patient’s natural lens to theoretically protect the eye from blue light.¹⁴ Although theoretically plausible, even the American Academy of Ophthalmology agree that the evidence is lacking to use blue blocking intraocular lenses.^15,16 However, UV spectrum blocking intraocular lens implants at the time of cataract surgery is indicated.¹⁷

Light and retinal damage measurements: time and brightness domains.

Light levels needed to cause photochemical injury to the retina is measured in both the time and the brightness domains.^4,18 Equations show that the maximum amount of diffuse light tolerable before retinal damage occur is at about 100 Wm^-2 (measurement of irradiance) with an exposure longer than 2.8 hours.⁴ It is impossible to determine the exact lux (measurement of luminance) of this level of irradiance, because of wavelength curves, but it is approximately the same as 120 000 lux.¹⁹ This is comparable to the diffuse irradiance of the sun at midday on a sunny day, at the equator.²⁰ Take note that solar irradiance is dependent on the latitude, the season and the atmospheric conditions.³ It is interesting to note that the aversion response (turning the head or closing the eyes) naturally protect us from light sources brighter than this.⁴ Looking directly at the sun intentionally (not the diffuse light measurements mentioned above) equates to about 1 220 000Wm^-2.³ The time for retinal damage with such an exposure is very short, in the range of seconds to minutes.³ Lastly, a meta-analysis of studies looking at sunlight exposure over a long period of time as a causative factor in age related macular degeneration, showed that there is no current evidence to suggest that sunlight exposure can be linked to the cause or progression of this disease.¹³

Wavelengths of light and the circadian rhythm

The colour wavelength of a sunny blue sky is measured at 475nm.²¹ The most potent wavelength for melatonin driven circadian rhythm suppression is measured at 446nm to 477nm.²² This means that when one is exposed to this blue spectrum light, then there is a suppression of the hormone called melatonin.²³ This of course is good during the day, but not so during the evening, if one wishes to have a traditional (or normal) sleep cycle. Melatonin is secreted to increase drowsiness and therefore drive the circadian rhythm by inducing sleep in the evenings when blue stimulation (sunlight essentially) is in the natural world at its lowest.^22,24,25 Morning exposure to light, in contrast to the evening exposure, moves the circadian rhythm forward.²³

A child’s circadian rhythm is about double as sensitive to light versus that of an adult.^8,26 This is due to the average pupil size as well as natural lens changes. Very importantly, another study showed that the pre- to mid-pubertal age group showed significant suppression of melatonin during evening light exposure compared to the late to post-pubertal group.²⁷

It is interesting to note that blue enriched light stimulation has been studied in positive mood modulation, attitude and work performance in office workers, and even as light therapy in psychiatric conditions, to name a few.^23,28 For example, light has been proven to be an anti-depressant and to slow down dementia. It is not the aim of this article to discuss the positive effects of light stimulation in more detail.

Conclusion

The wavelength of light most likely to cause retinal injury is of the shorter or blue spectrum; the even more harmful UV light is blocked by the natural human lens. There is no evidence proving that exposure to the blue wavelength of light is harmful to the eye with in vivo human studies. This includes exposure to LEDs and screens. One is quite unlikely to be exposed to the amount of radiant light needed to cause retinal injury voluntarily, as the aversion response will prevent this level of exposure. There is unfortunately no cure for forced sungazing and solar retinopathy burns. The circadian rhythm is driven by the blue wavelength through the expression of melatonin, at a peak wavelength of 446nm to 477nm. The blue sky has a wavelength of 475nm. Morning stimulation of blue light, and evening removal of blue light, is beneficial to the circadian rhythm. Children are more sensitive to evening stimulation of blue wavelength light than post-pubertal teenagers and adults and should therefore avoid blue light stimulation in the evenings.

Blue blocking spectacles may have a subjective improvement in eye comfort which cannot be measured, but apart from evening wear and circadian rhythm enhancement, there is no proof that blue blocking spectacles should be worn at all. On the contrary, it should not be worn during the day preferably. Current scientific knowledge, show that wearing blue blocking filtered spectacles or implanting blue filter intra-ocular lenses at the time of cataract surgery to protect your eyes, is a market driven hype rather than a scientific fact. If one feels subjective comfort by wearing blue blocking filtered spectacles, and sleep is not affected, then it will not harm the eyes physically to wear them either. The positive effects of blue light should always be considered.

References

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