Melanopic efficiency of light

The melanopic effect of lighting is rooted deeply in human development. Early in their development history, the entire life of humans, who had not yet become settled, was governed by available daylight. This light significantly determined phases of sleeping and waking, times for procuring food and times of retreat. Early humans already absorbed a great share of their environmental information through their eyes. This meant an advantage over other animal species at daytime, while in the dark of night, invisible dangers could lay hidden.

Only when fire became manageable, was it possible for humans to create cosy spaces which provided warmth and protection during dark hours. The first lamps and fireplaces were constantly developed further. While the oil lamps of the 19th century were predominantly used in noble residences, the more widespread gas lights gave rise to the first large-scale street lighting systems in major cities. Only the invention of electric lighting made home lighting more widely accessible. Using the new, affordable light source, days could be extended artificially, up to an entire illumination of nightly darkness. Mainly in the course of industrialisation, work processes could be uncoupled from natural daylight and factories were able to operate around the clock. The temporal self-determination facilitated by this was also reflected in society, which – particularly in the cities – no longer held to the rhythm provided by nature, but determined its own daily cycle.

These rapid societal and technical changes mainly took place over one single century – in evolutionary terms, too short a period for the human body to adapt to the new conditions. The connection to daylight and the rhythm provided by it, developed over thousands of years, was and still is a determining factor in the life of human beings.

The fact that daylight has a positive influence on humans has been known since the beginning of the 20th century and was applied, for example, in the architecture of sanatoriums as well as in sunlamp therapy.

Figure 1.39: Luminous efficiency V(λ) for the eye adapted to light (photopic vision) and V’(λ) for the eye adapted to darkness (scotopic vision) as well as the melanopic efficiency function Smel for circadian rhythms

Roughly since the middle of the 20th century, this influence has been the object of targeted scientific examination. The most famous scientists in this area are probably Colin Pittendrigh, the "father of the biological clock" and Jürgen Aschoff, who are widely regarded as the co-founders of chronobiology. The first observations in plants, insects and rodents eventually led to evidence for circadian rhythms in humans as well.

Since light therapy had yielded good results in treating seasonal affective disorder (SAD) and other deficiency symptoms, an interdisciplinary field of research developed, where behavioural scientists, biologists and physicians examined the effects of light on humans. In 1991, an additional photoreceptor not involved in the actual visual process but playing a key role for the circadian rhythm, meaning the 24-hour-rhythm of the inner biological clock, was discovered by Russel G. Foster and colleagues in the eyes of mice.

In 2001, such photosensitive ganglion cells were then also found on the human retina, which helped further explain the effects of light on the human day/night rhythm. Consequently, research into this area was intensified, and in 2007, melanopsin was determined to be the chemical agent in the ganglion cells. The effects of light on the human circadian rhythm has since been described using a spectrum of effects (see figure) on the inner clock which vary in strength depending on the wavelength of the light. However, direction and spatial distribution of the light also play a role in this. Extensive interdisciplinary research in the area lead to today’s solid state of knowledge of human needs and endangerments which can be induced by light.

Figure 1.40: The melanopic efficiency of light depends on the light’s angle of incidence and the surface dimension of the light source.