UVA and UVB rays

You might have seen this distinction on sunscreen bottles: apparently, there is not just “one” kind of solar radiation – there are actually two different components to it. UVA rays, which make up 95% of solar radiation and can enter the middle layer of your skin, are lower-energy. They are responsible for the skin’s premature aging after prolonged sun exposure. Importantly, UVA rays can get through glass and clouds. 

UVB rays are portrayed as more dangerous; they have higher energy, affect only the outer layer of the skin, and majorly contribute to various skin cancers. Luckily, most sunscreens protect from both UVA and UVB rays, so if you apply yours regularly, you should be in the green. There are also benefits to controlled exposure to both of these spectra – we’ll discuss it further. 

Well-being: SAD and physical benefits

Seasonal affective disorder (SAD) is one major way in which lack of sunlight (and daylight in general) manifests in our health. We all feel somehow “worse” when late fall comes around: the daylight hours become shorter, signalling to our bodies that a colder season has arrived. The thing is, daylight is one of the factors that regulates the production of melatonin (the circadian rhythm regulator discussed below) and serotonin (widely known as the happiness hormone – and a precursor of melatonin). That’s why we tend to sleep longer (or have our sleep cycle disrupted) and be moodier during the winter. 

Unfortunately, even though our body’s rhythms are still wound up to stay closely in sync with nature, the rhythms of our lives at work or university rarely slow down come November. And to the pile of uneasy sleep and general lethargy adds the feeling of “I am not doing enough.” To find out how to cope with SAD, we reached out to ITMO’s psychologists – discover their best tips in this article.

Apart from being linked to our mental well-being, there are studies showing that UV radiation can be beneficial for physical health, as well. For example, a 2019 experiment with mice fed a diet high in fat and sugar demonstrated that a weekly 30 minutes of solar radiation was associated with lower fat accumulation and the production of potentially beneficial triglycerides in the liver. There have also been hypotheses that sun exposure can contribute to cardiovascular health in humans. Both ideas, however, seem to be in need of further proof. 

One way controlled sunlight (at specific wavelengths) or artificial light is proven to be beneficial in medicine is in treating various skin conditions, from acne to dermatitis and even cancer. Phototherapy, a fancy term for this approach, has also been used to treat and prevent SAD, as well as non-seasonal depression and other mental disorders. So it isn’t for nothing that we tend to gravitate towards cozy lamps and fairy lights in the winter time (even if they aren’t necessarily UV ones).

Vitamin D synthesis

This point seems to be a little more controversial: on the one hand, sun exposure helps produce vitamin D in the skin, which is crucial for the health of our bones and immune system. Vitamin D deficiencies are associated with various kinds of disorders, proving the immense importance of this substance to our health. 

On the other hand, there is, of course, the now very highly publicized link between sun exposure and higher risk of skin cancer. Currently, the recommendations for sun exposure are as follows: “It is optimal to have sun exposure for 5 to 30 minutes a day, most days a week, without sunscreen as SPF ≤8 may block the body’s ability to absorb the UVB rays to effectively make vitamin D.” For longer periods of time, putting sunscreen on is important to lower the risk of skin cancer. So, as with many things in life, this one has to be about balance and moderation. 

Circadian rhythm

Circadian rhythm – our body’s “internal clock” that measures when it’s time to sleep or be awake, is regulated by melatonin. When we are exposed to sunlight (or bright artificial light) soon after waking up, in the evening the melatonin in our body is produced sooner; this allows us to fall asleep quicker and contributes to a good night’s sleep. Even on overcast days, you may get enough light for this effect to kick in: as we now know, UVA rays still reach the earth through the clouds; and there’s even been research stating UV radiation can increase as light passes through clouds. So, remember that sunscreen and don’t look directly at the sun without sunglasses.

Research into circadian rhythm is a fascinating story. Just one memorable anecdote from the book Why We Sleep by the sleep scientist Matthew Walker involves Dr. Nathaniel Kleitman and his assistant Bruce Richardson. As their own “guinea pigs,” in 1938, they spent a whopping 32 days in Mammoth Cave, Kentucky, in complete darkness and surrounded by all kinds of recording equipment, to see if their bodies would maintain their sleep cycles. Unsurprisingly for us now, they did, demonstrating along the way that the body’s clock doesn’t measure exactly 24-hour intervals. That is why we need sunlight (or daylight) to adjust it.

Evolutionary importance

Having, of course, contributed to the evolution of all life on Earth, the sun has had a major effect on human evolution, too. Geneticists have been looking into the precise components of solar radiation that have had the greatest effect on human evolution. They pinpointed that climatic ambient temperature has played a major role in human development and immunity among global populations. Current research into the matter concerns, for instance, the effect of adaptation to different daylight duration on our immune system, as well as attempts to identify the effects of climate adaptations on a genetic level – involving human genome data. It’s intriguing how many factors have contributed to make us who we are today! 

Find out more fascinating research with our tag Mind-Blowing Research of the Month.