Diurnal and seasonal Rhythms
Insolation of sunlight is the driving force for all life on earth. Due to declination it varies for a given place over a cycle around the sun and gives the seasonal rhythm. Another important timer is the rotation of earth around its axis. Only on the side facing the sun radiation occurs and the rhythm of day and night evolves. Over millions of years life on earth has adapted to these conditions.
Actinic complex in Bioclimatology
The full spectrum of the sun is considered in the actinic complex of the atmospheric impact complex. While short wave radiation of UV as well as long wave radiation of IR is well considered in our research the visible spectrum in between is neglected. We call it visual' why don't we take a look at it? Is it because we think we know everything about it?
Adaption to rhythms
Of course we do have knowledge about adaption of plants, animals, and humans to natural rhythms of light:
In plants we have those which blossom when days are short and those which blossom when days become longer. Most flowers open their blossoms during daytime, but there are some blooming during nighttime (like Nachtkerze). Famous botanist Peter Lenné made a flower clock, indicating each hour by another plant opening its blossom.
The niche of day and night for main action we also find in animals. We have night active insects as well as we have night active birds and mammals which use the darkness to hide from their predators. Which, in turn, evolved in that way that their senses either adopted to the faint light of moon and stars or that they improved those independent of light, like scent or hearing. Also well-known is the dependency of hibernation and reproduction to the rhythms of natural light.
What do we know about human being? We can count ourselves to the day active mammals with vision being the strongest sense for orientation. Only recently it was discovered, that in the human eye next to rods and cones (for vision) another type of cells exist which signal the body via hypothalamus whether it is day or night outside. In darkness melatonin is set free and swamps all over the body resetting the internal circadian clock. These cells can be compared to the third eye' of reptiles and fish.
As we are day active and as our strongest sense is vision and in turn it only works properly in brightness, we fear darkness. Human ability to solve problems learned to brighten darkness artificially and about one hundred years ago we invented electric light. In another aspect, we became independent of nature and its rhythms like we did before in thermal aspects or in agricultural aspects.
Still: Life is adapted to natural cycles of brightness and darkness, and changing its rhythm has an impact on well-being.
Known effects of artificial light
Plants
increased growth but what about thermal resistance? What about nutrition content? What about strength, e.g. resistance to wind? In fall, trees next to a street lamp keep their leaves on branches next to the light. When frost hits, these branches are not prepared and are damaged.
Animals
Salmon in Norway: artificial light at night suppresses hormonal development: animals gain weight but do not reach adulthood. Thus, they don't feel the need for migration or to fight for revier but they stay peaceful within the group.
Hamster in Israel: In two fields the same number of animals was put. Aim of the study was to test whether light at night was a possibility for pest control. By influencing their hormonal system it was expected to suppress reproduction. However, the experiment was over-effective as none of the animals on the lit field survived winter. Obviously, artificial light at night also influenced their system of thermal regulation.
Cattle in Brazil: Due to high temperatures during daytime animals are phlegmatic and hardly show appetite. To increase their weight, food and water is also offered during night. Compared to a dark food station a lit one is more attractive and more frequented.
Hamster in lab: While one group of animals was provided a normal rhythm of day and night another group of animals was kept in a rhythm with artificial light at night. The latter ones changed their rhythm of food intake and became obese compared to their mates with normal light schemes.
Whether these effects of artificial light at night were wanted ones or not why do we think that they have nothing to do with ourselves?
Human
Indeed, medical research has begun pointing toward adverse effects of artificial light at night on human health. Artificial light at night (ALAN) was brought into discussion as causation for cancer in 1987. Since then the relationship was investigated for different types of the disease: in women focus was on breast cancer while in men focus was on prostate cancer. Whereas the knowledge about the impact of indoor artificial light at night is fairly advanced and shift work was declared as carcinogenic by WHO in 2007, insights into the health-effects of outdoor artificial light in the sense of light pollution (e.g. outdoor illumination shining in through bedroom windows) is still in emergence.
Also, there are hints that artificial light at night influences metabolism and thus plays a role in epidemic of obesity.
Conclusion
In Bioclimatology we are interested in the wavelengths of UV and IR why do we ignore the part in between? In Bioclimatology we are interested in natural thermal conditions outdoors as well as in more artificial thermal conditions indoors why don't we consider those aspects of light in our research? With this presentation I would like to start a discussion what contribution we can give as bioclimatologists to the topic of effects natural and artificial light has on life on earth.
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