In the course of human development, the genetics of mankind have adapted to these astronomical circumstances and linked inner processes to the day-night rhythm. Thus human beings are tired and unmotivated at night, for their bodies are waiting for the sleep that will help them to recover. In the morning, when they wake up, their efficiency increases significantly and reaches its highest point between 10 and 12 o’clock. 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.
In the hypothalamus of the brain, the detected light stimuli are used to regulate the biological clock. 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 suitable light during the day disrupts the hormonal synchronisation of the biological clock and can lead to insomnia, reduced motivation and moodiness. 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.