This presentation was held during the LightSymposium 2016 in Wismar, Germany.
Recording date: October 14th, 2016.
Abstract: Humans vitally depend on environmental light conditions. Light intensity, spectral composition and timing influence literally every function in the organism, regardless if of conscious or autonomous nature. The adaptation to the outer world is crucial for survival and survival is fundamentally provided by vegetative adaptation. Mediators and directors of these autonomous functions such as circulatory, metabolic and water balance are hormones and neurotransmitters. Photoendocrinology describes how the environmental light conditions, the “natural chronology of the cosmos” (Hufeland), translate into the rhythmical vital functions providing diurnal activity and stress in alternation with nocturnal restoration and regeneration.
The 19th century marked the dawn of photochemistry, which represents the scientific base of photobiology. Every photobiological process starts with a primal photochemical reaction, followed by a chain of consecutive responses. Photoendocrinological reactions can be found in all kinds of living multicellular organisms and are not exclusively reserved to humans. Many hormones involved in photobiological reactions have been developed in an early stage of evolution and have been preserved ever since (The Science of Photobiology 1977).
Hormones involved in photobiological reactions are small molecules responsible for chemical signal transduction and unfold their biological activity either via the bloodstream (endocrine) or in the direct vicinity of their formation (paracrine). Hormones usually act in very low concentrations by initiating preprogrammed cellular processes which are triggered when the hormone molecule binds to its specific receptor. Three major classes of hormones exist: eicosanoids, amino acid derivatives and steroids. The latter are cholesterol derivatives and play a paramount role in many photoendocrinological processes. Aromatic amino acids are the precursors of many important signaling molecules involved in photobiological reaction chains (Nussey & Whitehead, 2001).
II. Natural and artificial light and its effects
Natural and artificial light sources differ significantly with regard to intensity, spectral distribution, electromagnetic properties and timing. Human metabolism and autonomous functions are complexly adjusted to the environmental light conditions with the principal purpose of ensuring survival. Human skin is – in contrast to the majority of other mammals – unprotected by fur, which is remarkable especially under the circumstance that humans originated from the equatorial regions of Africa with high solar irradiation levels (Jablonski & Chaplin, 2000; Juzeniene, Setlow, Porojnicu, Steindal, & Moan, 2009). This has far-reaching consequences for local and systemic photoadaptation processes, which are mainly, but not exclusively, regulated by ocular light perception. The skin contributes to photoendocrine reactions e. g. via nitric oxide (Juzeniene & Moan, 2012) and vitamin D photosynthesis (Baggerly et al., 2015).
Photoadaptation processes involve skin protection, water balance, acid-base regulation, energy metabolism, thermoregulation, reproductive functions, immune system reagibility and motor activity, to name only the most important (Hollwich 1979). Besides these predominantly autonomous regulations, photoadaptation in a highly complex manner involves also psychological and cognitive aspects (Juzeniene & Moan, 2012).
Exposed to the natural light source with the highest impact, the sun, all these photoadaptation mechanisms (Fig. 1) are executed in a meaningful manner, comparable to the coordinated interplay of musicians in a philharmonic orchestra, which is directed by the conductor. While sunlight with its broad spectrum, high intensity and complex variance and timing is the true maestro, artificial light sources are just surrogates of a higher principle which is untouchable even by contemporary high tech.
Many lighting professionals seem to be fascinated by the possibilities of digital lighting technologies, which provide high energy efficiency, all rainbow colors and color temperatures, ruggedness and versatility. But – with regard to biocompatible lighting - true mastership becomes evident if not every available possibility is used: Biologically effective artificial lighting is controversial (Arbeitsausschuss NA 058-00-27 AA 2013; International Commission on Illumination 2015; Kommission Arbeitsschutz und Normung 2015) and authentic human centric lighting is much more than suppressing melatonin and collectively manipulating workers with photons.
Truly understanding the complex human photobiology should rather lead to deep respect for the inner harmony of the individual human and to the industry´s quest for biologically neutral artificial lighting.