Why Sunlight is Key to Overall Health
Sun is the driver of all life on Earth.
Hide your shades, hide your sunblock. It’s time to get some sun!
Our modern society seems to fear sunlight. Everyone wears sunglasses even when it’s cloudy and people slather on sunblock due to an irrational fear of sun cancer. More on that later. We’re always inside and are rarely in sync with the rising and setting of the sun. This has detrimental effects on our health. At a more esoteric level, we are just beings of light. At a physical level, we are indeed light and need light to live.
We need sun exposure not just for vitamin D3 production, whose health benefits you can find in my paper here, but also for a myriad of other aspects of our health. We seem to understand that plants need proper sunlight, but we haven't seemed to connect the dots to human health in modern society.
I started thinking more about our sunlight exposure and circadian rhythm when we started daylight savings time in Spain last Sunday. I felt so disoriented for two days, even though it was only a time change of one hour. I noticed it was darker in the morning, which made it harder to wake up, and lighter in the evening, which made it more difficult to fall asleep early. It turns out a study conducted in Europe concluded that daylight savings time leads to higher rates of cardiovascular disease, so I knew I was onto something, and most chronic disorders can be linked to circadian rhythm disruption and improper light exposure.
Light Wavelength matters
The sun provides the planet with a full spectrum of light ranging from UV rays to infrared light and is a major source of electromagnetic energy, which is just a fancy way of saying that energy comes in different colors, some of which we can and cannot see. For example, humans cannot see infrared or ultraviolet light.
The other important thing to know is that different wavelengths of light have different lengths which can penetrate tissues at different depths. Long wavelength light like red light has an amazing ability to penetrate tissues like deeper layers of our skin and maybe even into our bone marrow for example, compared to short wave light. If that wasn’t cool enough, light can penetrate certain organelles in our cells like mitochondrion and the nucleus. In short, light is the cheapest and most effective way to change your health.
UVB comprises a very small percentage (around 5%) of total UV light from the sun, but is more readily absorbed by our body and is very effective at affecting our biology such as aiding in the production of beta-endorphins, vitamin D, and serotonin. UVA rays, on the other hand, are absorbed more in our eyes.
Basic eye physiology
Light affects every single cell in our body, but not all light can reach our cells. However, when light is absorbed, it affects the photoreceptors which can then trigger various cellular responses which can affect other organs in our body. Light is a big communicator about what’s going on in our environment.
We have photoreceptors in our eyes (retinal pigment epithelium) that regulate our circadian rhythm, hormones, and neurotransmitter production (serotonin, dopamine, melatonin). These photoreceptors in the back of our eyes (rods and cones) absorb light. Rods absorb light but aren’t very picky about the color of the light (think light and darkness). The cones either absorb red (long wavelength), green (medium wavelength), or blue light (short wavelength).
The contraction of the pupil is controlled by the sympathetic nervous system whereas the dilation of our pupils is controlled by the sympathetic nervous system. So if our nervous system isn’t regulated, the amount of light that enters our eyes could be off which has a negative downstream effect.
Intrinsically photosensitive retinal ganglion cells have a blue light detector that sends signals to our brain that regulates our circadian rhythm. When we are constantly exposed to blue light from screens and artificial lighting, it negatively affects our circadian rhythm. This unopposed blue light, compared to the blue light that’s naturally found in full spectrum light from the sun that contains many wavelengths of light, produces photo stress on our eyes. It destroys photoreceptors and dissociates photoreceptors from vitamin A. When there is too much vitamin A in the retina, ruins photoreceptors and causes problems with processing natural sunlight. Adequate UVB exposure that hits these cells on the other hand also stimulates the turnover of stem cells in your skin, hair, and nails.
A small percentage of UV light can penetrate our eyes without being filtered from the cornea and lens. There is a biological reason for this. We are biologically designed to receive natural light from our environment. The idea that we have to avoid all sunlight contact on our skin and eyes is false.
Finally, 50% of your blood volume travels through your retina and they’re responsive to UVA light. UVA causes nitric oxide release which increases blood flow to the eye and the brain. This prevents hypoxia, which is low oxygenation, in eye cells and allows us to absorb more photonic energy, and prevents retina deterioration.
Our eyes are the master of our circadian and circannual rhythm
The eye has direct communication with the HPA axis through the autonomic nervous system. This means if you don’t get enough natural light exposure, or conversely, if you’re exposed to too much artificial blue light, you’re stressing out your body. Also, a light and timing mismatch, such as shift work or staying up too late at night, also creates biological chaos which eventually leads to inflammation Besides light stress, psychological stress can also cause an increase in adrenaline which keeps the pupils dilated, which can cause light sensitivity. I hope you’re starting to see how all these pieces are connected.
Many will argue that the hypothalamus is the master clock of our circadian rhythm, but ophthalmologist Dr. Jay Montgomery argues that the eyes are the master clock because these organs are the first to absorb light and send those signals to the brain. Within the hypothalamus is the suprachiasmatic nucleus which communicates directly with our nervous system and the optic nerve. When the sun rises, the sun emits infrared and blue light. This kickstarts the circadian rhythm and the HPA axis, such as a spike in cortisol which you want to be high in the morning and taper off throughout the day.
Besides a circadian rhythm, we also have a circannual rhythm. This makes sense because the days are longer in the summer months and shorter in the winter. Our body knows this— the intrinsically photosensitive retinal ganglion cells communicate with our pineal gland so that it can either create or shut down melatonin production depending on the amount of light exposure. So on short days when there is less light landing on these cells, the duration of melatonin release is longer, and vice versa in the spring and summer months.
Healthy Sun exposure promotes skin, metabolic, and immune health
Before you all throw your sunblock at me, hear me out. I’m talking about moderate sun exposure, not sunbathing until you burn. Our skin is our biggest organ and serves as a type of switchboard between our external (i.e. sun) and internal environment.
The skin contains a type of cell called chromophore, such as melanin, which act as photoreceptors that absorbs different wavelengths of light and sends signals throughout the body that regulate the following:
Increased ATP production (i.e. energy) which increases fibroblast proliferation, and collagen and cytokine production that aids in wound healing.
Vitamin D3 production, which promotes bone, immune, and brain health, and likely more that scientists have yet to discover.
Promotes a healthy skin microbiome, which leads to healthy skin AND acts as a natural sunblock.
Increased nitric oxide production which increases blood flow.
Regulates stress via the HPA axis
Involved in hormone production (thyroid hormones, steroid hormones, serotonin, catecholamines, prolactin, melatonin, etc).
Regulates various neurotransmitters and beta-endorphins
Stimulates anti-inflammatory and immunosuppressive pathways and communicates with immune cells like cytokines, all of which promote healthy skin and prevent conditions like dermatitis and psoriasis.
Plays a role in metabolism by regulating appetite and thyroid hormones.
The eyes affect melatonin production
Melatonin is a topic worth a post all on its own, but to summarize: It’s a light-sensitive hormone and a potent antioxidant that is best known for regulating how asleep or awake we feel, but it’s so much more.
It can positively affect bone mass by stimulating osteoblastogenesis, which is bone production and promotes overall bone health through various mechanisms.
It regulates sexual maturation by regulating gonadotropin-releasing hormone and/or gonadotropin secretion. High levels of melatonin can suppress gonad mass and ovary function. Kids have a naturally high level of melatonin, which is a good thing, so we can start to understand how poor sleep habits in pre-teens and teenagers negatively affect their sex hormones.
Melatonin activates our immune system. UVB exposure helps regulate our nervous system and our spleen function. So this means during the winter months we need to make an effort to get sunlight exposure, or at least UVB light. This will keep our nervous system regulated and the downstream effect is regulating our immune system so that we can properly combat infections. Wound healing is also better with proper UVB exposure.
The mitochondria produce melatonin, which triggers autophagy and apoptosis among other health benefits. Check out my post about light exposure and mitochondrial function here.
Sunlight exposure affects sex hormone production and fetal wellbeing
A study in humans and rats found that 20-30 minutes of midday UVB light exposure, caused improved follicle maturation, which correlates with improved fertility, as well as an increase in sex hormone levels (beta-estradiol, progesterone, and testosterone) in the summer months. This makes sense to me because it’s been well documented, at least anecdotally, that ovulation is affected by our circadian rhythms such as traveling to different time zones or poor sleep habits.
Studies have also shown that melatonin and a stable circadian rhythm positively affect maternal, fetal, and placental physiology, largely due to its antioxidant properties. Melatonin also regulates placental development. Studies have also shown melatonin plays a role in healthy fetal development. Women with placental insufficiency have been found to have lower overall and placental melatonin production as well as lower expression of melatonin receptors in the placenta.
I don’t know about other mothers out there, but my OBGYN never told me about the importance of prioritizing healthy sleep habits… Sigh.
Sunlight exposure also enhances mood and cognitive function
We all instinctively know that sunny days make us feel happier compared to cloudy days. Regardless if you have seasonal affective disorder (SAD) or not, proper sun exposure improves our mood and reduces depression. A study also showed that adequate sun exposure leads to improved cognitive function.
There’s a part of our brain (the parihabenular nucleus) that receives signals from our eyes when we have light exposure, especially UVB light, that triggers the production of serotonin and dopamine in the brain. The eye’s retina also produces dopamine and serotonin which have various physiological effects on proper eye health and function. BUT, if this pathway gets activated at the wrong time of day, our mood worsens and our dopamine production decreases. The key is sunlight exposure at the proper time of day (first thing in the morning and throughout the day and little light exposure at night and definitely no UVB exposure from 10 PM - 4 AM).
Tips for healthy Sun exposure
Increase your sun exposure gradually especially when transitioning from winter to spring.
Gradually reduce the amount you wear sunglasses, except when skiing or for long periods at the beach which do cause light stress on the eyes. Over time your eyes will become accustomed to being “naked” and sunlight.
Avoid long periods of sun exposure from 12 - 3 PM in the summer.
Stay hydrated.
Eat a balanced diet. Phytochemicals from vegetables (especially those high in beta carotene and lycopene) and fruits like blueberries and watermelon are skin protective and reduce photoaging.
Consume adequate essential fatty acids for skin and overall health.
Avoid excessive showering and aim to wash your “pits and bits” to protect your skin’s natural oils and to ensure adequate vitamin D3 production. Yes, you can basically wash off a lot of the precious vitamin D from your skin.
Use natural sunblock whose main ingredient is zinc oxide to protect your skin and the coral reef. Brands I like are Thinkbaby and Badger Skin.
Download the app Dminder which sends you a notification of when to get out in the sun based on your current location and angle of the sun.
Tips for Sleep and melatonin production
We need an appropriate amount of sunlight daily that’s dependent on the season. In the spring and summer, we should try to get outside as much as possible, and in the winter it makes more sense to stay inside.
First thing in the morning: don’t turn on the lights or look at your phone. Instead, get outside and get natural light right into your eyes as the sun is rising whenever possible. Obviously this is more difficult in the cold, dark winter months or when it’s cloudy.
During the day: Try to get mid-morning and mid-afternoon sunlight exposure. Use blue light-blocking filters and glasses in the evening.
Night time:
For optimal melatonin production and better sleep: dim lights, use long wavelength lights in your house, use light-blocking blinds, and avoid looking at screens, or at least use a blue light filter, and keep the room cool.
Light exposure (like a bathroom light) in the middle of the night can cause melatonin to fall way down rapidly when it should be high. Try to use the minimal amount of light possible if you have to get up at night.
Be careful with melatonin supplements, especially in kids and pregnant women. As previously mentioned, melatonin regulates placental growth and suppresses sex hormone production in pre-pubescent kids, which is normal. Supplements often contain doses that are far higher than what we naturally produce. Fix your sleep habits and daily sunlight exposure first before supplementing.
Sources
https://www.sciencedirect.com/science/article/pii/S1567724916302586
https://www.sciencedirect.com/science/article/pii/S1011134422002287
https://www.sryahwapublications.com/archives-of-ophthalmology-and-optometry/pdf/v1-i2/3.pdf
https://www.sciencedirect.com/science/article/pii/S0944501322000842?via%3Dihub
https://www.sciencedirect.com/science/article/pii/S2211124721010135
