It is amazing how much we depend on the most self-evident things in our natural environment. Almost everyone is familiar with the expression winter and spring depression. The reason for feeling low at that time of the year is simply a lack of sunlight. In Scandinavian countries with their long dark winter months, this phenomenon is even more pronounced than in Germany.
How come the influence of the sun on our well-being is so important?

With an average of 1400 g, the brain is only 2% of the total weight of our body, but it consumes a quarter of the inhaled oxygen. A lack of oxygen for a few seconds can already cause fainting. The brain also consumes 20-25 % of our total energy. (Some biologists think this high energy expenditure is the reason why the big advantage of intelligence for survival in evolution was effective so late.) Because of this high metabolic rate, it is logical that the extremely complicated brain is much more sensitive to changes in metabolism than our other organs. Light has a huge impact on our metabolism, and thus also on our thoughts and feelings.
Lack of light, by the way, is an important cause of bone diseases such as Rickets or osteoporosis. We know that calcium and vitamin D are important components of the bone structure. The vitamin D in food is, however, inactive and is only converted to Vitamin D3 with the aid of sunlight on the skin. Only this vitamin D3 is able to produce the important effect on our bone metabolism.
Therefore, any sunny winter day is a therapy of nature for people suffering from osteoporosis or a tendency to wintry gloom. They should be outdoors as much as possible, since often a lack of physical activity happens on bleak winter days.
Since these correlations were detected, light therapy is an integral part of medicine. Light as a therapy is used for mental or physical disorders. Even skin diseases such as eczema get better in a Mediterranean climate.