Light and health

How does light affect your health? Is there a difference between sunlight and artificial light sources? Which impact has blue and green light on your vision and circadian rhythm? Continue reading to find the answers to these questions.

What is Light?

The electromagnetic spectrum ranges from low frequency, low energy radio waves to high frequency, high energy gamma rays. The human eye percieves visible light (or just light) ranging from around 380 nm to 780 nm of the electromagnetic spectrum. The spectrum of visible light is continous with no clear boundaries between one color and the next. Speaking of colors, we often refer to the colors of the rainbow the visible light is divided into violet, blue, green, yellow, orange and red. Daylight is white light composed of all of these colors. Violet and blue have the shortest wavelengths and contains more energy than light of longer wavelengths. Violet and blue light is sometimes referred to as high energy visible light. 

Beside the visible spectrum the sun also emits radiation in the ultraviolet and infrared regions of the electromagnetic spectrum. Daylight is a dynamic light source that varies depending on location, time of day, season, weather and atmospheric pollution conditions.  Throughout the day ultraviolet, infrared and visible light increase in intensity before peaking in early afternoon. These intensities drop significantly by late afternoon. Sunlight contains less than 10% uv light, 43% visible light, 42% infrared A, the rest is infrared B and C which is percieved as heat. By blue light we mean the blue wavelength portion of the light spectrum which ranges from 450-495 nm wavelengths. Green light ranges from 495-570 nm.

Artificial Light Sources

We encounter artificial light sources everyday in our lives. Light sources that are both broad-spectrum white light and colored light produced from a narrow range of wavelengths. These light sources can be found in a wide range of applications, for instance household lighting, televisions, computer and phone screens and street lighting. White light sources vary in their color. They are generally percieved as ranging from a warm red-yellowish-white light to a cooler/brighter blueish-white light. 

To describe the percieved color from a light the industry uses correlated color temperature (CCT) measured in Kelvin. A low CCT generally, but not always corresponds to a relatively low proportion of blue wavelengths. As CCT increases the appearance becomes a cooler blueish-white color. Incandescent lamps and halogens produce a warm light weighted towards the yellow, orange and red end of the spectrum. More energy efficient technologies including compact fluorescent lamps and LED are available in both warm and cool colors.

At a given CCT the spectral distribution can vary quite a bit. With incandescents and halogens you pretty much know what you get. The spectral distribution of these light sources has a small variation. With LEDs we have a different story. Two LED light sources with the same CCT can have different spectral distributions. To distinguish between the two you need a spectral power distribution chart. This kind of chart will give you the proportions of the different wavelengths emitted. From the spectral power distribution charts below you can see that modern lighting like fluorescent lamps and LEDs contains little to none red and infrared light. This makes them different from other light sources. Incandescent and halogens have a spectrum power distribution which is closer to the one of the sun regarding red and infrared light.

Light Detection in the Eye

We have known for quite some time now that the retina of the eye contains cells called rods and cones which contribute to image formation. The rods are responsible for the black and white or monochromatic vision. Rods detect light between approximately 400 nm and 600 nm. The cones on the other hand is responsible for the color vision. The eye has three types av cone cells – small(S), medium (M) and large(L). The maximum sensitivities for the small, medium and large cone cells are in the blue, green and red regions of the color spectrum. They have their peak sensitivity at approximately 420 nm, 530 nm and 560 nm respectively. 

Rods and cones were for a long time thought to be the the only light detectors in the eye. In 2002 another type of cell called Intrinsically Photosensitive Retinal Ganglion Cells, ipRGC for short, was found. These cells drive non-image forming responses to light, such as the pupillary light reflex and modulation of sleep, alertness and activity. IpRGCs directly respond to light via a blue light sensitive chemical called melanopsin, which has a peak sensitivity of around 480 nm. They also receive information from other wavelengths indirectly through interconnections with rods and cones. IpRGCs is not involved in image-forming but instead sends signals to the hypothalamus of the brain. These signals affect processes like circadian rhythms and the interaction between the nervous system and hormones of the endocrine glands.

As people age their eyes will transmit less light to the back of the retina due to yellowing of the lens. The eye becomes less responsive to all wavelengths of light with age, with shorter wavelengths in the violet and blue regions more prominently affected.

Eye Damage

Momentary exposure to high intensity light sources can result in severe and permanent damage to the retina. Retinopathy is an injury commonly associated with gazing directly at the sun or a solar eclipse. Arc welding or high-powered blue colored lasers are examples of blue light sources where this is an issue. Blue light levels used to induce this damage are much higher than the retina would experience under normal circumstances. Retinopathy is not an issue using artificial light sources for general purpose lighting.

The health risks from direct viewing of blue enriched light sources vary and depends on viewing duration and intensity. Digital eyestrain is a new term used to describe the conditions resulting from the use of today’s popular electronic gadgets. Digital eyestrain is a medical issue with serious symptoms that can affect learning and work productivity. Symptoms of digital eyestrain, or computer vision syndrome, include blurry vision, difficulty focusing, dry and irritated eyes, headaches, neck and back pain. Enriched blue light is shown to increase the risk for macular degeneration.

Effects of Blue and Green Light on Circadian Rhythm

The suprachiasmatic nucleus in the hypothalamus region of the brain, contains the bodys master clock. It synchronises the internal daily rhythm to the environmental cues given by the external day and night cycle. To be more specific, it uses light and absence of light for this synchronisation. The master clock synchronises daily rhythms at both organ and cellular level. How light affects the human body depends on timing and duration of exposure, brightness, as well as its spectral content. The circadian rhythm influnces many processes in the body including metabolism, immune function, sleep and other aspects of behaviour and mood. The master clock recives input exclusively from ipRGCs in the eye. These cells are predominantly influenced by blue light at high intensities, although they also receive information from other wavelengths via interconnections with rods and cones.

Exposure to light, particularly blue wavelengths, at an inappropriate circadian phase leads to circadian disruption and related health and behavioural consequences. Our increasingly 24/7 lifestyle alters our patterns of exposure to light and directly challenges our circadian drive for sleep at night. Exposure to blue wavelengths in the evening, including from domestic lighting and light emitting screens, delays the circadian clock. This interference makes it harder to fall asleep at night, to wake up in the morning, and impedes attention abilities the next morning.

Shift work results in people sleeping and working at sub-optimal times of the day. This leads to poorer sleep and health, reduced productivity, and increased risk of errors and accidents. Shift work is sort of a non voluntary circadian misalignment, whereas social jet lag is a voluntary one. Social jetlag, which is a difference in sleep patterns between weekdays and weekends leads to circadian misalignment with the solar cycle because the body receives inconsistent exposure to daylight and light at night.

The fact that the circadian system adjusts to light exposure is a good thing. This means that the body can adapt to the changing day and night cycle over the course of a year, but also that we can adapt to the jetlag we experience travelling across time zones. These adaptations to a new environment is however slow. The length of a day typically change only a few minutes per day and adaptation to new time zones takes several days. It seems it is a bad thing to constantly be out of phase regarding the circadian phase. It seems to be of little importance regarding health if you are misaligned by a constant shift in your wake and sleep cycle or if yor are misaligned by constantly shifting your wake and sleep cycle.

The fact that the circadian rhythm adapts to a new environment is somehting that we can use to get a better circadian alignment. The rhythm can be reinforced with blue-enriched white light at an appropriate time to improve alertness, performance, mood and sleep quality. Natural exposure to blue light from the sun during the day is a good way to achieve this. The intensity of blue light is greatest around midday. Exposure to blue light in the morning will advance the circadian clock. This will help people who wants to move their sleep to an earlier time. Outdoor activities, including camping under natural light conditions, can increase exposure to high intensity light during the day and re-calibrate the circadian rhythm. Exposure to bright daylight may also reduce the sensitivity of the circadian system to light exposure at night compared to those experiencing dim daytime light with minimal outdoor light contact.

As mentioned above, as people grow older less blue light is transmitted to the retina. This lower level of transmission during the day is thought to negatively affect sleep for elderly people. Older people generally need stronger signals to reset the master clock. Younger people however are more sensitive to blue ligth and sets the circadian rhythms more easily. 

Equally important as getting bright light during the day is that is dark during the night. Repeated flashes of saturated blue light (480 nm) during the circadian night, even delivered through closed eyelids, has been shown to shift the human circadian clock. Blue and green light at night affect the quality of sleep and lower the production of melatonin. Melatonin is a very potent anti-oxidant, which assists in the repair of the mitochondria while you are asleep.

This text is an adaptation of the publication: Blue light – Impacts of artificial blue ligth on health and the environment. 
https://royalsociety.org.nz/assets/Uploads/Blue-light-Aotearoa-evidence-summary.pdf
Licenced under Creative Commons 3.0 New Zealand