The Sun, UV Light, Vitamin D And Your Health

It’s that time of year again for us in the northeastern US. We can finally go outside without shivering. Oh Joy! I am planning on being outside all day as it is going to reach 73 degrees. It got me to thinking that now I can make my own vitamin D. A bunch of other thoughts popped into my head about the sun, its seasonality, sunburn, vitamin D, diet and health. After seeking the internet for a bit, I got the answers I was looking for. I am going to share them with you now.



In temperate northern latitudes, we tend to have hot summers and cold winters. For those of you who are not astrophysicists, this is because the earth’s rotation is not at 90 degrees straight up and down relative to the sun. We rotate on a 23.5-degree angle. This means that the north is angled away from the sun in winter and towards the sun in the summer, hence the seasonal changes. The earth is closer to the sun in winter and farther away in the summer since the sun is not centered in the middle of our orbit. This makes our winters milder and our summers cooler than in the southern hemisphere! Another effect of the rotational angle is longer summer days in the north. Amazingly, people living near the equator have little variation in daily sunlight throughout the seasons.

UV Radiation

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The sun produces all sorts of electromagnetic radiation. The atmosphere and the earth’s magnetism protect us from most of the harmful effects. Visible light and UV radiation are the one’s most familiar to us. UV radiation is broken up into three categories: A, B and C. UVA are longer wavelength rays responsible for tanning. They account for 95% of all UV that hits us. The rays penetrate deeper into the skin causing damage. Our response is to produce melanin to absorb the rays before they cause further damage. When you have enough new melanin, you appear tanner. UVB is shorter wavelength that damages the surface of the skin since it can’t penetrate as far as UVA. This is the wavelength responsible for sunburns and vitamin D production. UVC does not reach us because the atmosphere absorbs 100% of it. Since we are angled towards the sun in summer, we get much more of these rays. (1) The amount of UV can be calculated on a daily basis. This is useful, as it varies due to the seasons, cloudiness, and time of day. It is called the UV index and can be found on most weather apps.


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Courtesy of

Courtesy of

The sun starts to get stronger as winter progresses. Here in Connecticut, we still don’t need to be concerned about overexposure when early spring arrives. The reason is that the UV radiation is not strong enough yet. In April, the average UV index is 3 and this is the intensity that can cause us to burn. Those of us with light skin are much more prone to burning in April because we have no tan left from last year. People with darker skin are much less prone to sun damaged skin. The Fitzpatrick Skin Type Index is a great tool to help you determine how to deal with the sun. (2)

You should never burn your skin. The best way to avoid a burn is to get out of the sun prior to turning pink. You will have to determine your skin type and the UV index for that day. In Australia where the sun is stronger, they say Slip (on sun protective clothing), Slop (on SPF 30+ sunscreen or higher), Slap (on a broad-brimmed hat), Seek (shade), and Slide (on wrap-around sunglasses).

Vitamin D

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Sun exposure is the easiest way to get your vitamin D. Vitamin D is formed in skin after exposure to ultraviolet-B radiation (UVB), which enters the bloodstream and undergoes hydroxylation in the liver to 25-hydroxyvitamin D (25(OH)D). This is the major circulating form of vitamin D and is used as a measure of vitamin D status. This is further hydroxylated in the kidney to 1,25-dihydroxyvitamin D (1,25(OH)2D), which is the active form. (3) The trick is to get enough sun to make vitamin D without burning. Luckily, the lighter your skin, the quicker you make vitamin D. Most people will have adequate vitamin D levels just from incidental exposure – even outside the peak UV times of 11am and 4pm. In fact, it is thought that as little as five minutes per day of incidental sun exposure (such as walking outside to the mail box or to catch the bus) is enough for someone who burns easily, and up to 20 minutes is sufficient for a person with darker skin. (4) For this reason, i the sunnier months, we don’t have to rely on our diets to get our vitamin D if don’t want to.


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If you get the proper amount of sun, you will have stronger teeth, bones, better sleep, improved mood, and a healthier immune system. Serotonin is one of the feel-good neurotransmitters. The rate of production of serotonin by the brain has been directly related to the prevailing duration and strength of bright sunlight and rises rapidly with increased brightness. (5) In winter, our work schedules can make us rise in the dark and receive too little sun to get into a normal sleep-wake pattern. This leads to fatigue, forgetfulness, sleep disorders, and even depression. Seasonal Affective Disorder (SAD) is prevalent when vitamin D stores are low. (6) Studies have suggested that low levels of vitamin D are associated with poor mood in general. There are a number of trials that have suggested a role for Vitamin D in the supplementary treatment of depression. (7)The longer, brighter summer days helps alleviate these symptoms. You can read more about this in my previous post here. Sedentary lifestyles are also associated with lower levels of serotonin, anxiety, depression, and weight gain. The warmer, longer days make it so much easier to be active. My gardening chores alone are enough exercise to make me feel noticeably better.