The term "circadian rhythm" is defined as a biological rhythm with a length of roughly 24 hours (circa = roughly, dies = day). A typical circadian rhythm therefore is the human sleep-wake rhythm.
Circadian rhythm and inner clock
As early as the 1950s, Gustav Kramer and Jürgen Aschoff researched the sleeping behaviour of people kept in artificially illuminated rooms for several weeks under isolated conditions without contact to daily progression or daylight rhythm. Their sleeping behaviour was compared with that of other subjects which had been living under normal, daylight-influenced conditions. While the latter group routinely slept between 9 p.m. and 7 a.m., the sleeping behaviour (meaning the need for sleep) changed completely after a few days under isolated conditions. The periods of falling asleep and waking up had shifted daily (see figure). After about 21 days, the subjects slept from 4 p.m. until about 1 a.m. at night. After only a few days, a significant temporal shift in the sleeping rhythm had already taken place.
For these temporal shifts to be corrected in relation to the time of day, the inner clock must be synchronised with the time of day. Light is the strongest indicator of time in this regard. The ambient light is used to synchronise the inner clock through the aforementioned ganglion cells. The effects of this synchronisation can be described using the progression of natural daylight, which was the only light-time indicator in evolutionary terms. Particularly bright light at noon, for example, can prevent tiredness in the afternoon (see section ). White light with an increased blue portion or light with a high colour temperature corresponding to the diffused light of a blue daytime sky can lead to increased alertness and attention. In the evening, this effect can also be utilised to achieve increased alertness in people despite advanced hours.
In a working environment, light with an increased blue portion can prevent sleepiness during the day and at the same time support a more restorative night sleep if it is preceded by relaxing, subdued light and darkness in the evening. On the other hand, unwanted waking phases can also be induced by light, for example when a bathroom luminaire with increased blue portion is turned on at night, which can lead to short-term sleeplessness.
Biologically, these processes work by release or suppression of certain hormones (melatonin, cortisol, serotonin etc., see section "Melatonin") jointly responsible for tiredness, stress or performance capability. The hormone release of various glands in the brain is mainly triggered by the photosensitive ganglion cells which makes the influence of light on the hormonal balance directly traceable through hormone concentrations present in the blood.
In figure, examples of circadian rhythms are displayed for:
cortisol levels, which increase at the start of the day and increase human activity and thus alertness. For this reason, cortisol is often also referred to as a stress hormone.
human attention as a consequence of the cortisol levels
melatonin levels, which increase strongly at night/in the dark and are reduced during the day. Melatonin is therefore also referred to as a sleep hormone.
body temperature as a result of the organism’s activity. The progression of melatonin levels in the blood divides the 24-hour day into a biological day (workday, ergotropic phase) and a biological night (rest day, trophotropic phase) (see also section ).
Sleeping problems and even illness can occur when the inner clock is thrown off balance artificially and over longer periods of time. An example for this is constantly varying shift work which can cause permanent jetlag due to receiving light at work during the night and artificial darkness during the day thus leading to health issues. A shift such as this can be re-synchronised by a properly adjusted light progression.
The term "melanopically effective light" describes the temporal, relative variation of characteristic physical and photometric values. Particularly the temporal variation of intensity and spectral composition of melanopically effective light can synchronise the "inner clock". Disruptions which can throw the "inner clock off its beat" and thus render the synchronisation ineffective must be avoided as far as possible.