No other environmental factor is so controversially discussed as solar radiation. While dermatologists warn against strong UV radiation - against early skin aging, free radicals and skin cancer - the fear of vitamin D deficiency is increasing, especially in winter.
The reason for this discussion is the ultraviolet radiation of the sun, which reaches us through the earth's atmosphere every day.
A distinction is made between different wavelengths. The shortest wavelength of radiation within the UV spectrum, i.e. the most energetic, is UV-C radiation, followed by UV-B and UV-A.
The Biochemistry behind it - simply and radiantly explained
Although UV light is not visible, it is a powerful form of energy that interacts with our bodies. The energy of radiation is the starting impulse of many chain reactions within the body that can affect our health positively or negatively.
Above all, radiation can pass its energy to organic molecules, which change their configuration as a result. Since the form of a molecule determines its function in biological systems, the body suddenly has something new in hands.
The sunniest vitamin
The best example of the positive effect of solar radiation is the synthesis of vitamin D.
The word vitamin suggests that it is a vital substance (lat. Vita = life) which is not produced by the body itself. This is only partially correct. Because in most human skin cells the vitamin precursor 7-dehydrocholesterol is waiting for its destiny to be transformed into Vitamin D3 through sunlight.
Vitamin D forms to remember
UV B radiation with a wavelength of 290-315 nanometers can convert the molecule into the biologically active vitamin D3.
This results in a more flexible molecule that leaves the plasma membrane and enters the blood with the help of the vitamin D-binding factor. The vitamin later enters the liver, where it is converted into calcidiol, the storage form of D3.
In the kidneys and other organs, calcidiol is further converted into calcitriol. Only this form binds to the vitamin D receptor in the cells. This receptor then can bind the DNA and through this switch is able to regulate your metabolism.
Are there more reasons to spend time in the sun?
Yes. And no.
Not for vitamin D. Experts particularly recommend short, intensive periods in the sun with as much free skin as possible. For people with light skin, this can mean between 10-20 minutes  if UV radiation is optimal. This means: During summer when the sun is at its highest point and there is little cloud formation.
In darker skin, more of the pigment melanin is stored, which protects against UV radiation but also limits vitamin D formation. Therefore, the required sun time is slightly longer.
A short but intensive sun exposure produces a similar amount of vitamin D as longer sun exposure, but here, the damaging consequences predominate. This is due to the fact that the finished vitamin can be destabilized by too much light. The appropriate time should be determined according to skin color, time of day and season.
There is another reason why light affects our health. The Circadian Rhythm.
The bodies of most animals are designed for a binary daily routine. We sleep at night and are active during the day. The two times of day are distinguished by light. Our body perceives light stimuli and transforms them into hormonal signals that make us tired or awake and set some other things in motion.
A balanced day-night rhythm promotes health.
The "night hormone" melatonin plays a crucial role in that.
It is formed from the neurotransmitter serotonin, produced during the day, as a response to darkness. In winter, for example, dark times predominate over bright ones. Therefore there is more melatonin than serotonin. It is thus assumed that this is the cause of winter depression . It's dark, melatonin predominates and the serotonin level, which influences positive mood, decreases.
While blue light suppresses melatonin synthesis , red light stimulates it. Before going to bed, red light is therefore ideal for balancing one's own day-night rhythm.
That's why the "night-mode" makes your screens red.
If the sun is so important, is winter dangerous for me?
In the winter months, the formation of vitamin D is reduced due to the low UV-B content in sunlight. North of the 51st degree of latitude, it can even be absent at times.
Who can develop a deficiency? 
- People in polar and subpolar regions who don't eat fish
- People with darker skin in northern areas with lower UV-B radiation
- People at a distance from the equator in winter
- elderly people who are no longer able to produce enough of the precursor molecule
While in the period of industrialization 90% of children showed undesirable physical development due to vitamin D deficiency , this danger is averted today. At that time, the naturally low UV radiation in Europe combined with cloud formation and industrial air pollution often caused rickets, bone malformations and tuberculosis.
Today, however, we're creating part of the risk ourselves by working and spending our free time indoors.
8 hours in closed rooms - health risk?
So, what about your working hours at the office?
As far as lighting is concerned, it has been shown that windows in the working environment are legitimized by more than just personal preference. They influence the emotional and psychological balance of an employee, according to a 1998 study .
The same study found general lighting to be secondary. Important is the size of the areas covered by sunlight and the view. These were directly linked to employee satisfaction, general well-being, and stress reduction.
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 Johansson, C.; Smedh, C.; Partonen, T.; Pekkarinen, P.; Paunio, T.; Ekholm, J.; Peltonen, L.; Lichtermann, D.; Palmgren, J.; Adolfsson, R.; Schalling, M. (2001). "Seasonal Affective Disorder and Serotonin-Related Polymorphisms". Neurobiology of Disease. 8 (2): 351–357. doi:10.1006/nbdi.2000.0373.
 Brainard GC, Hanifin JP, Greeson JM, Byrne B, Glickman G, Gerner E, Rollag MD (August 2001). "Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor". J. Neurosci. 21 (16): 6405–12. doi:10.1523/JNEUROSCI.21-16-06405.2001.
Holick MF. Vitamin D: a millennium perspective. J Cell Biochem 2003; 88:296–307.
 Leather, P., Pyrgas, M., Beale, D., & Lawrence, C. (1998). Windows in the Workplace: Sunlight, View, and Occupational Stress. Environment and Behavior, 30(6), 739–762. https://doi.org/10.1177/001391659803000601