The biological significance of light

In the course of human development, human genetics have adapted to astronomical conditions and internal processes have become fixed to the day-night rhythm. We are therefore tired at night and less likely to perform, as our bodies are programmed to sleep for regeneration. In the morning, our ability to perform increases when we wake up and reaches its peak between 10 am and 12 noon. The early afternoon is characterised by an energy slump, which is then followed by a second increase in efficiency in the later afternoon. In the evening hours, the body comes to rest again and once more prepares for the sleep phase.

However, man´s biological clock does not follow the length of the day-night rhythm exactly, but constantly has to be synchronised with the environment. Among the reasons for this are the varying genetic preconditions of human beings (everyone has a different biological clock) and the variable length of day and night in the course of the seasons. These processes are controlled by non-visual perceptions of light and are examined in chronobiology; here we also refer to the melanopic effect of light.

It was not until 2002 that photosensitive ganglion cells were also found in the retina of human beings, besides the known photoreceptors (cones for colour vision, rods for mesopic vision). These do not serve visual perception, they have high sensitivity to blue light and also control the pupillary reflex, for example. Light stimuli detected in the hypothalamus of the brain are used to regulate the circadian rhythm. For example, the non-visual perception of light there inhibits the secretion of the hormone melatonin, which normally makes people tired and initiates the sleep phase. It is assumed that the high level of blue sensitivity in this process expresses a strong orientation towards daylight.  Insufficient exposure to light during the day disrupts the hormonal synchronisation of the circadian rhythm and can lead to sleep disturbances, mood swings and a lack of motivation. In central and northern latitudes, this phenomenon occurs more often in winter and can have a depressive effect. 

This problem can be countered with circadian illumination adapted to the needs of the biological clock. The objectives here are to promote well-being and to simulate natural daylight conditions as far as possible. To achieve this, light with a chronobiologically highly effective blue component is used in the phases of increased performance, while in the recovery phases in the evening performance is reduced by lowering the blue content and increasing the red components in relation to each other, thus supporting the initiation of the sleep phase. In practice, such illumination is usually achieved through the use of luminaires with variable colour temperatures.

Spectrum of daylight · Wavelength [ nm ]

For many living beings, the sensory perception of light is the most important sensory function. Humans are no exception here, as we literally live by what we can see. The human eye is relatively highly developed and is considered to be the primary sensory organ. For almost all activities, human beings need an adequate level of light. If this is not available, the primary organ of sensory perception cannot be used and we find it difficult to find our bearings, to move and to work. For this reason, the most important forms of work for man relied on daylight for many thousands of years. It was only with the invention of artificial light sources (pine chip, torch, oil lamp) that man’s performance also increased, as for the first time the hours of darkness could also be used productively. It was only then that humans started making enormous progress in terms culture, science and technology.

One of the core tasks of illumination with artificial light is to create and support optimal lighting conditions. Proceeding from the capabilities of the human eye, therefore, a visual task has to be defined and then solved as well as possible with the means of lighting technology. Aspects like physical comfort and well-being must be taken into accountin addition to the simple task of "seeing". Another aspect to consider is the ability of the human eye to adapt to lighting conditions. This can take the form of constant contrast ratios within the lighting situation, for example. The nature of the artificial light also plays an important role. The spectral composition of the artificial light must correspond as closely as possible to that of sunlight if the visual task is to be the assessment of individual colours. Furthermore, the composition of the light also has a direct influence on man´s biological clock and thus also on his efficiency and well-being.