The Human Circadian System

The Human Circadian System is fundamental to our physiology and controls much of our behaviour. Circadian derives from the words circa dies meaning "about a day" and is linked to the cyclic system where the human body adapts to the rhythmic of active and passive periods (for humans that is the day respectively nights) over a period of circa 24 hours.

Recent research has found that the Human Circadian System consists of numerous endogenous clocks, of which the brain's clock is recognized as the "Master clock" (the suprachiasmatic nucleus = SNC). There are clocks in virtually all cells and organs of the human body, and the main function of the Master Clock is, like the conductor of an orchestra, to ensure all other clocks are in tact (synchronized). However, the Master Clock, even being referred to as a very precise clock, is not able to generate rhythms equivalent to the astronomical day of precisely 24 hours.

Generically we are all equipped with different lengths of the endogenous clocks, meaning some people have an internal rhythm being slightly shorter than 24 hour and some slightly longer. In modern terms this phenomenon is defined by our chronotype, most commonly divided into two types - morning and night persons (also often referred to as Morning Larks and Night Owls). Morning Larks tend to get up early in the morning and go to sleep early at night. Night Owls show the opposite behaviour.

Since our circadian rhythm varies from the astronomical 24 hour day we need to synchronize the circadian system every day otherwise we would soon start to feel the effects of the desynchronization in terms of symptoms similar to the ones we know from e.g. jetlag or working on night shifts.

The most powerful external stimulus for synchronizing the circadian system to a 24 hour cycle is exposure to a robust light/dark cycle, this signal is mediated through the eyes to the SNC. When humans are exposed to a daily dark/light cycle, physiological mechanisms and behavioral patterns through the SCN are aligned and synchronized. Hereby the organism are able to fine tune behavioral patterns and demands, sleep and activity. Does this not happen it can leed to sleep problems, fatigue, hormonal and methabolic problems.

Being of different chronotypes and thus with different circadian rhythms we have different needs for light exposure over the time of day. Typically you will find a significant difference between a Morning Lark and a Night Owl of up to several hours in the release of e.g. the sleep hormone - melantonin. This is the reason, that Night Owls are often peaking much later than the Morning Lark during the day in terms of e.g. performance and subjective feeling of vitality.

When humans are exposed to a daily dark/light cycle, cyclic production of specific hormones influences e.g. sleepiness and alertness, body temperature, metabolism etc..

Blue enriched white light exposure in the morning can influence different types of hormones and neurotransmitters by e.g. increasing the cortisol level (stress) or serotonin (impulse control) and decrease melatonin (sleep). Reduction of bright light exposure at night allows for the production of melatonin and growth hormone and will help support a sound deep sleep and thus optimizing the regeneration of our body and mind over night.

“Age, gender and chronotype are some of the chronobiological elements that scientists have indicated play an important role in deciding what type of light we need, and when and for how long we need it.

Meet Peter,

he is a Night Owl, which means his circadian rhythm is characterized with a late start in the morning. Peter typically feels ready and vitalized later than his oppositions - the Morning Larks. A light boost in the morning (bright white light) can help him get synchronized to a modern life working cycle.


on the other hand, being a different chronotype and also being older, has different needs for personal light settings over the time of day than Peter. Age-related changes to the human eye may disrupt the circadian response and numerous research results indicate that e.g an older adult (+60 years old) may need up to 3 times more light and higher intensity than that of a younger (+20 years old)*.

Ref: * Van Someren, E.J. (2000). “Circadian rhythms and sleep in human ageing.” Chronobiol Int. May,17(3).

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