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Natural light health benefits for solar powered humans

Public health messages focus on the hazards of too much sunlight because of the risks of skin cancer with less emphasis on the flip side of the coin: sunlight as an indispensable element of good health. In this article, we dive into the flip side, whilst acknowledging the importance of striking the right balance.

As Jorg Reichrath points out in his introduction to Sunlight Vitamin D and Skin Cancer, there’s a consistent association between malignant melanoma and short-term intense UV exposure, particularly burns acquired in childhood, and protection against UV radiation is an integral part of skin cancer prevention. However, chronic, less-intense UV exposure hasn’t been found to be a risk factor for melanoma. In fact, it has been found in some studies to be protective.

Health benefits of sunlight

We need natural light in contact with our eyes and skin. This is regardless of the weather, in order to synchronise our biology appropriately to the outdoor environment, the seasons, and the daily cycles of light and dark, upon which our sleep, hormonal and metabolic function, mood and alertness, and cognitive performance depend.

Like plants, humans need sunlight. We photosynthesise 90% of our vitamin D in our skin and vitamin D deficiency is linked to many chronic diseases, paradoxically including cancer. Rather than too much light, overall, we’re exposed to less natural light as we spend more time indoors.

Inadequate environmental light (or light reaching the retina in the eye and brain) can cause circadian disruption, increasing the risk of insomnia, numerous medical conditions, depression, and possibly early mortality. We also need natural light to maintain visual health and good eyesight.

Spending time outdoors has been shown repeatedly to have beneficial effects for vitality (having both physical and mental energy).

A Swedish 20-year study of 30,000 women found that sun avoidance is risky and of a similar magnitude to smoking for all cause mortality, particularly in countries with low-intensity light exposure.

What is natural light?

Natural light is the radiation coming from sunlight, which is typically made up of 53% infrared light, 44% visible light, and 3% ultraviolet light. Colour temperature is a way of measuring different shades of light by giving it a value for how ‘warm’ or ‘cool’ it is. It’s measured in Kelvin (K) with red/yellow colours at the low end of the scale and the blue shades at the high end.

A graphic that shows what natural light is by showing different sun icons at different heights in the sky against different coloured backgrounds that correspond to the type of light most abundant at that time of day. The colours start off red as the sun rises, fading to yellow in the early morning, white in the late morning, blue midday, white in the early afternoon, yellow in the late afternoon then red as the sun sets. A scale at the bottom shows the colour temperature throughout the day measured in Kelvin (K). It starts at 1,800K at sunrise, moving to 4,000K mid-morning, 5,550K late morning, 8,000K just after midday, 12,000K in the afternoon and 16,000K at sun set.

The composition of natural light is constantly changing over the course of the day. Red light heralds the start and end of the day, with brighter blue light towards midday. Indoor lighting, on the other hand, is quite different in make-up and potentially suboptimal for supporting human health, performance, and wellbeing.

Light is characterised by its wavelength and frequency

A graphic showing the natural light spectrum in colours with different colours blocks joined by green arrows coming from a sun icon. At the far left is a dark purple colour with 'UVC' written in it. Next to that is a lighter purple block with 'UVV' written in it and next to that is an even lighter purple block with 'UVA' written in it. Then there are some blue, dark green, light green, yellow, orange, red and dark red blocks in the middle and right of the diagram. Above the purple blocks the word 'Ultraviolet' is written. Above the green, yellow and orange blocks in the middle 'Visible' is written and above the dark red block on the right of the diagram 'Infrared' is written. The wavelength of each colour in (nm) is written underneath. '200' is written underneath the 'UVC' block on the far left, then '290' is written under where the 'UVB' block starts, 'then '320' is written under where the 'UVA' block starts and '400' is written under where the 'Visible' blocks begin. '700' is written where the 'Infrared' block starts.

The sunlight that we see – i.e. visible light – is made up of a mixture of colours, each with a different wavelength within a range of 400-700nm. However, as the image above outlines, there’s light either side of these ranges so, what we see is not all there is to see. A nanometer (nm) is a billionth of a meter.

How does the body absorb light?

  • Skin
  • Eyes
  • Plants

Light is absorbed by light receptive pigments in our skin and eyes called chromophores and indirectly coming from plants. Some important chromophores you might have heard of include, melanin (in your skin and eyes) and haemoglobin (in your red blood cells and skin), melanopsin/vitamin A (in your eyes), and bilirubin (in your bile and blood).

Light absorption in your skin

Different frequencies and colours of light penetrate your skin to different depths.

Ultraviolet, violet, and blue light (because of their shorter wavelengths and higher frequencies) have more energy, and therefore greater potential to cause damage in excess. However, they’re able to travel less deeply compared to lower energy coloured and infrared light.

A diagram showing how different coloured natural light penetrates the human skin. Different layers of the human skin re shown including the pores on the top surface of the diagram, the epidermis, derma, fat and muscles at the bottom. Dead keratinocytes are labelled at the top surface of the diagram. Keratinocytes are labelled i the derma layer of the diagram. Basal cells are labelled in the line between the Derma and the fat layer. Fat cells are labelled in the fat layer and myocytes are labelled in the muscle layer. A black arrow labelled 'UV' points down to the top of the epidermis layer. A blue arrow labelled 'Blue' points down to the area between the epidermis and derma layer. A green arrow labelled 'Green' points downwards to the top of the fat cells layer. A red arrow labelled 'Red' points towards the bottom of the fat cells layer. A black arrow labelled 'NIR' points to the bottom of the muscle layer.

Most of the time when UV-B comes into contact with the skin, it’s blocked by the epidermis. It’s usually considered that only 10% of UV-B reaches the basal layer of the epidermis as opposed to 50% of UV-A, which reaches the derma.

Light absorption in your eye

Light is needed for visual and non-visual purposes.

Light entering the eye is the primary signal for circadian rhythms, which stimulates and regulates autonomic, metabolic and hormonal processes, such as the release of the stress hormone cortisol and hormone melatonin, body temperature control, blood pressure regulation, cardiovascular efficiency and muscle power.

Photoreceptor cells known as rods and cones absorb light and convert it into electrical information which we visualise as an image. Rods are important for vision in low light/night time conditions and cones are needed in bright light conditions, for colour vision and a clear image.

Light absorption by plants

Plants capture light in pigments in their cells, such as chlorophyll (green), carotenoids (bright yellow, red, and orange colours) and flavonoids like anthocyanins, which impart red, blue, and purple colours to certain vegetables, fruits, and berries. These photopigments may have anticancer, antioxidant, and anti-inflammatory properties.

When we eat a rainbow of different coloured fruit and vegetables we access the energy that has been captured and stored in that food and benefit from a myriad of properties they contain. Light filled food also includes blue green algae, such as spirulina and chlorella, which contain chlorophyll and carotenoid pigments.

Different types and different colours of light exert different biological effects

Wavelengths of light in the blue range and also bright white light (coming from artificial sources) have been shown to be alerting and to stimulate cortisol, the hormone of wakefulness. Blue light in the range 400–470 nm also has antimicrobial properties. Melanopsin, the hormone which tells your master clock it’s daytime, also absorbs blue light, which then inhibits the release of the multi-functioning hormone melatonin (best known for sleep promotion in response to darkness).

Different wavelengths of light and colours of light are/have been used therapeutically to treat a host of conditions including psoriasis, acne, tuberculosis, tuberculosis of the skin, wound healing, pain relief, hair regrowth, skin rejuvenation and neonatal jaundice.

Light in the red/near infrared range (600-950nm) used in low-level light therapy, has been shown to enhance energy production in the mitochondria (the energy powerhouse of your cells). It has also been demonstrated to improve cell signalling, growth factor synthesis, and reduce oxidative stress, which can cause damage to cells and tissues. Infrared light has been identified as having therapeutic value in treating a range of conditions, such as skin anti-ageing, wound healing, athletic performance, hair loss, fat loss and pain. You can read more about it here. How this happens exactly at cellular level and which light frequencies and “doses” of light are best for a purpose is unclear and needs more research to establish.

Natural light contains all these frequencies and therefore can have wide reaching potential health benefits.

Our daily lives can often introduce barriers to light absorption for example…

Normal window glass will allow most all UV-A light to pass through but blocks everything below 330nm, that’s most all UVB. Violet light can also be missing because there’s an overlap between its frequency range and the upper end of the ultraviolet (UV) spectrum. Some researchers think the lack of violet light could be contributing to the world-wide increase in myopia (short-sightedness). They also found that contact lenses which allowed in violet light, suppressed the worsening of the condition in a group of shortsighted children.

UVB light is intentionally filtered out because of its high energy and potential to do damage by:

  • spectacles
  • contact lenses
  • sunglasses
  • window glass
  • car windows

Can I get all I need from indoor lighting?

You’re unlikely to get all you need from typical indoor lighting. Indoor light is very different to natural light in terms of brightness and spectrum. Light emitting diodes or (LEDs), which we use in our homes, were designed for lighting and energy efficiency by physicists not biologists. They typically use 75% less energy because they don’t produce heat, which means infrared and red light frequencies are reduced or absent. LEDs replaced incandescent and halogen bulbs between 2009 and 2018 by regulation.

As highlighted above, natural light, by comparison, is made up of 53% infrared light, 44% visible light and 3% ultraviolet light.

6 line graphs, all with the vertical 'y' axis labelled 'Intensity' and the horizontal 'x' axis labelled 'Wavelength (nm)'. The first line graph shows those metrics for different colours of daylight. The 2nd line graph shows those metrics for different colours of fluorescent light. The third line graph shows those metrics for different colours of halogen light. The fourth line graph shows those metrics for different colours of incandescent light. The fifth line graph shows those metrics for different colours of cool white LED light and the sixth line graph shows those metrics for different colours of warm white LED lights.

Indoor light is less bright, typically no more than 500 lux in a well-lit classroom, and unlikely to exceed more 2,000-3,000 lux in indoor work areas, unless done so intentionally.

Natural light contains more than 10,000 lux (less on overcast days). A bright sunny day can reach over 100,000 lux.

This difference in brightness matters; a series of studies using fluorescent (tube) lighting found that over 500 lux can improve measures of concentration and reading comprehension compared to current standard lighting.

Who might benefit from more light?

In one study of care home residents it was reported that exposure to more than 1000 lux during the day could slow the rate of cognitive deterioration of people with dementia, as well as reduce feelings of depression. Elderly people in care are commonly getting much less than this. For example, it was shown in one study that the median light level that residents were exposed to was only 54 lux with just 10 minutes a day spent in light over 1000 lux.

It’s known that circadian photo-reception in our eyes declines progressively with ageing, making the elderly especially vulnerable to the ill effects of inadequate light. Therefore, should they lack adequate light exposure, it’s all the more important to address. According to Turner and Mainster: “Light deficiency, whether due to improper timing [in other words, inappropriate light and darkness for the time of day] suboptimal spectrum or insufficient intensity, may contribute to medical conditions commonly assumed to be age-related inevitabilities.

Bright light (using various regimens and artificial sources) has been used successfully treat seasonal affective disorder (SAD). The best studied protocol uses more than 2500 lux for two hours per day but equally brighter light at 10,000 lux 30 minutes per day could be just as good.

There’s solid evidence that exposure to brighter light can reduce the risk of short-sightedness, (although factors, such as light frequency/colour, may also have an impact, as highlighted above). Some Australian researchers say that children need to spend three hours per day under light levels of 10,000 lux to be protected against myopia.

Natural light in a nutshell

Rather than too much light, overall, we’re being exposed to less light and a limited spectrum of light as we spend more time indoors under electric lighting, behind electronic screens and windows. We see less natural light during the day and less darkness during the night.

Frequencies of light found in full spectrum natural light are being used therapeutically for all sorts of conditions. Light therapy commonly uses low level infrared light (frequencies that are entirely missing from indoor lighting). Windows remove most ultraviolet B frequencies and violet light.

Natural light contains all these frequencies, and therefore can have wide reaching potential health benefits.

Indoor lighting is recognised as being potentially suboptimal for supporting human health, performance, and wellbeing.

Substantial evidence indicates that altered light exposure contributes to negative impacts on health, sleep, productivity, increased risk of falls, and an increased incidence of heart and metabolic disorders like diabetes, Alzheimer’s and forms of cancer.

We need to get outside every day, whilst taking care to avoid getting sunburnt when it’s bright and sunny. Health agencies recommend limiting time in the midday sun, wearing protective clothing in strong sunshine, applying sunscreen and noticing the daily UV index.

Natural light: recommendations

Reviewed by:

Anna Keeble MA BA Wellbeing Expert

Dr Claire Marie Thomas MRCGP DFSRH DTMH DipNLP MBChB BMedSci Medical Expert

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Nicky Verity

Nicky Verity is a wellbeing researcher at Evergreen Life. A former clinical pharmacist, Nicky is passionate about empowering others to help themselves.

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