Can light impact our biology ?
Everyday, everywhere stories from the lens of science
Every evening when we look out from our balcony we see this view (Figure 1), it is spectacular, isn’t it! If you are curious like me, you will notice and wonder why some homes have red, amber or green coloured lights ?
Don’t you bother…I’ve done the hard work for you all. Read this article till end to know the answers! Article too long, just go directly to Key Takeaways section at the end!! Happy Reading and Learning :)
Physics and Biology of Light ?
In Physics, light is described as an electromagnetic radiation (EMR) which has both properties of particle and wave. The behaviour of EMR depends on its wavelength. Higher frequencies have shorter wavelengths and lower frequencies have longer wavelengths (Figure 2). Our eyes can only perceive light in the range of 400–700 nanometers (nm), which is referred to as the visible light. Light of wavelength below 400nm (like, gamma rays, x-rays and ultraviolet radiation) and above 700nm (like radio waves, microwaves, infrared radiation) are not visible to human eye.
Light whether visible or not can alter the function (for better or worse) of every cell in the body. This impact can be either direct — light sensed by the photoreceptors of the eye or absorbed by melanocytes of skin. (Melanocytes are melanin pigment producing cells and are located in the deepest layer of skin) or indirect —that is to say, light hits photoreceptors then starts a domino effect to downstream organ or cells. However, all these effects start with a particular wavelength of light being absorbed.
Light of short-wavelength (e.g., blue, green, UV light) is easily absorbed by the top surface of the skin but can’t penetrate deep tissue easily. However, longer wavelength light (e.g., red, infra-red) can penetrate way beyond the top layer of tissue and maybe even down to bone or bone marrow.
This post is dedicated to visible light and its impact on our biology. Follow up post will explore the impact of Ultraviolet, Infrared, X-rays, gamma rays and microwaves. Stay tuned!
Visible Light Spectrum
Visible light is a spectrum of 7 colours of different wavelengths. When a beam of white light travels through a prism, it disperses into this beautiful spectrum of Violet, Indigo, Green, Yellow, Orange and Red (VIBGYOR ). We all have seen this phenomenon in nature and we generally refer it as a Rainbow!
Sun is the main source of natural light on Earth. The light of sun which reaches Earth is in the ranges of ultraviolet, visible, and infrared radiation. Other sources of light are fire, artificial light sources like electrical lights which comes in many forms, e.g. incandescent, fluorescent, or light-emitting diode (LED).
Our vision starts with eyes. Each eye has a lens, much like the lens of a camera, that takes all of the light that comes from the environment and focuses it onto the retina of the eye, which lies a short distance behind the lens. In the retina there are two kinds of photoreceptors: rods and cones which are responsible for vision. Rods function at low light levels and cannot mediate colour vision. Cones, by contrast are active at higher light levels, can see colour (baring colourblindness) and absorb light of different wavelengths. Depending upon their sensitivity to different range of wavelengths, there are 3 different types of cones in our retina: S-cone (short-wavelength) M-cones (middle-wavelength), and L-cones (long-wavelength). Besides rods and cones, human eye contains one more specialised photoreceptor in ganglion cells (group of nerve cells that connect retina to brain) called intrinsically photosensitive retinal ganglion cells (ipRGCs). They contain photopigment melanopsin and are intrinsically sensitive to light, peak sensitivity is in the blue wavelength range (Figure 3).
Surprisingly, rods and cones demand the most energy out of all the cells in the body.
Science of Vision
Eyes capture the light and convert it into electrical signals which via retinal ganglion are sent to brain for interpretation. The image formed on the retina is an inverted and an unprocessed 2-dimensional one, it lacks depth, height and distance. It is brain which then deciphers and decodes the pattern of electricity received from eye and creates a meaningful image. All this processing happens in just fraction of seconds. No wonder why brain uses 40-50% of real estate for vision. It can be said that we are not actually seeing objects directly - we are just making a guess!
Effects of light on our Biology
The body takes in information about light and slowly changes the way biology works. Effect of light on our physiology can be rapid, moderate or at slow pace. Even the slow integrating effects of light can cause powerful and long lasting changes in our biology.
To understand the rapid effects of light on our biology, imagine this, if you are moved from a dimly lit room to a very brightly lit room - immediately you will feel very alert! You can try this at home. What happens in the above scenario is that bright light absorbed by photoreceptors of the eye gets converted to electrical signals snd sent to a region in brain stem called locus coeruleus (a nucleus in the pons of the brainstem involved with physiological responses to stress and panic) which will then trigger the release of adrenaline. Gush of adrenaline will then send immediate wake up signal to the mind and body. This response is actually hardwired in our system.
Now, let’s understand the moderate and slow integrating effects of light on our mind and body.
1. Circadian Rhythm
Light is the primary regulator of human circadian rhythms (24-hour cycles that are part of the body's internal clock). Every cell and organ in our human body has its own biological clock which is under the control of a master clock located in hypothalamus region of the Brain. Master clock is basically a group of 200,000 nerve cells called suprachiasmatic nuclei (SCN) which receives direct input from the eyes. Your eyes communicate information about the time of day to the rest of the body. Low vision or no vision you likely still have retinal ganglion cells which signal time of day to the brain.
As soon as light hits our eyes, melatonin, a hormone secreted by pineal gland (a pea shaped region in the brain) that makes you sleepy is acutely suppressed. When there is less light—for example, at night—the SCN tells the brain to make more melatonin so you get drowsy. On average, melatonin is an indicator of how much light is in the environment. This oscillating levels of melatonin remain in sync with the natural light-dark cycle and controls our wake-sleep cycle.
However, this balance can go off track and can shift the biological clock with just a brief pulse of light during night. Short wavelength light, between 470 and 525 nm (i.e., blue light) has the most potent effect on decreasing night levels of melatonin. Phones, laptops, tablets (mostly all electronic screens) are primary sources of blue light.
2. Circannual Rhythm
Our body also keeps track of where we are and in which part of world by measuring the average melatonin levels in the brain and body throughout the year (Isn’t this remarkable!). This phenomenon is called circannual rhythm. In short days, duration of melatonin release will be longer, whereas in summers when days are longer, the duration of melatonin release will be shorter. In this way melatonin is a communicator of how much light in average we have in our environment. In gist, our body has a calendar system that is based on the hormone which uses light to sense where we are on earth.
Besides controlling circadian and circannual rhythm of our body, melatonin also have powerful effects on other functions: melatonin increases bone mass, it also has negative effects on gonadal function, it regulates the timing of puberty. High levels of melatonin can actually decrease testis function and suppress the maturation of follicles. It can have potential effects on developing foetus and placenta. It also has potent anti-oxidant effects, can activate our immune system and anti-cancer effects. This doesn’t mean that taking endogenous melatonin supplementation or staying in dark will magically give you all the beneficial effects of melatonin. It is the rise and fall of melatonin or the oscillating levels in sync with the circadian rhythms which is required to enjoy all the good melatonin has to offer.
3. Sleep
Dark period increases melatonin which makes us sleepy. Sleep is essential for our well being, it is during sleep, our body restores the immune, nervous, skeletal, and muscular systems, these are vital processes that maintain mood, memory, and cognitive function.
Light exposure during the nighttime sleep period could facilitate the misalignment between the master clock and clocks in metabolic tissues, which will impact glucose metabolism and insulin levels. Light exposure during night will also have arousing effects on the sympathetic autonomic system, which will increase cortisol levels (stress hormone), also increase heart rate and decrease heart rate variability. Thus, only a brief exposure of light during sleep can disturb the deep sleep, heart health and metabolic functions. Chronic exposure of light during sleep will increase your chances of getting obese, metabolic disorders and cardiovascular diseases.
4. Mating Behaviour
Melatonin suppresses gonadal axis, decreases testosterone and estrogen hormones in both males and females and if light inhibits melatonin, light directly can regulate the mating behaviour. There is another parallel pathway which controls mating behaviour and interestingly it is also regulated by light. However, this is not the light absorbed by eye but by our skin and light in the ultraviolet B range. Recent study reported that UVB light exposure to females enhanced their fertility and chances of getting pregnant by increasing egg maturation, follicle growth and sex hormones. When human males were exposed, their testosterone level increased. Both males and females reported increase in romantic passion after UVB exposure (Figure 4).
UV exposure in the morning and day is important but avoiding light exposure at night is equally critical.
5. Mood
Light also can control mood and level of alertness as well. Bright light at the wrong time in the 24-hour cycle can make your mood worse by decreasing dopamine “happy hormones” levels.
What we do with this information, you may ask ?
Key Takeaways
One of the best thing you can do is to get proper amount of sunlight to your eyes each day, appropriate to the season. Spend more time outside during summers and spring and its perfectly fine to spend more time inside during fall and winters. Our bodies are tuned to seasonal change.
“If you are not viewing sunlight for 2-10 minutes in the early part of the day when the sun is low in the sky and doing the same thing early in the evening, you are severely disrupting sleep rhythms, mood, hormones, metabolism, pain threshold, and many other factors including ability to learn and remember information.” – Dr. Andrew Huberman
Avoid shortwave light (blue, UV) exposure between 10 pm and 4 am – or even 8 pm onward if you suffer from depression. Avoid screen time completely before 1-2 hour of sleeping. If on some days you can’t avoid completely then shift to night-mode screen lighting, wear glasses with blue light filter lens or wear glasses with amber or red coloured lens.
At dusk try to change your environment by either dimming the light or use long wavelength lights (amber or red coloured lights). If you may recall that melanopsin cells are very sensitive to short wavelength blue light and can suppress melatonin.
Ah! Finally, now we know why some apartment windows have red/amber light! (Figure 1)
If you wake up in the middle of night to use washroom, avoid using bright fluorescent lights, use the minimum amount of light required to safely move through. Bright light will plummet the melatonin levels to nearly zero or zero which will disturb the sleep and circadian rhythm.
Avoid light during the sleep cycle.
Tip for shift workers or people awake at night for childcare, etc.: if you need to be awake at night, red light is the best choice because it will not inhibit melatonin or increase cortisol
Even just one night of dim light exposure during sleep (even if it doesn’t wake you up) may impair cardiometabolic function such as increased insulin resistance and decreased heart rate variability
Avoid melatonin supplementation!
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