The human biological clock is controlled via the light

Light is the most important “zeitgeber” to control our circadian rhythm. Daylight is therefore an important contribution to our wellbeing. Cool white wavelengths in the light supresse melatonin secretion.
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​The human visual and hormonal system. Light enters the eye and signals are sent from the retina to the brain's visual centre and suprachiasmatic nucleus. The most efficient light exposure angle for maximum ganglion cell triggering is from above the horizon.
Source: Licht.de
 


 
Scientists have been studying the biological impacts of light for decades. But it was not until 2002 that they discovered ganglion cells in the retina that are not used for seeing. The newly identified cells respond most sensitively to visible blue light and set the biological clock that synchronises our bodies with the external cycle of day and night.


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The human eye retina contains three photoreceptors: colour sensitive cones, dim light sensitive rods, and blue light sensitive ganglion cells.



A major output of the biological clock system is the production of the hormone melatonin – a “sleep hormone”. This production in the pineal gland varies with the time of day. Melatonin is secreted at night and has minimal levels during daytime. A larger melatonin suppression, triggered by light exposure, often coincides with increased feelings of alertness and higher sustained attention.

The hormonal pulse generators

The ganglion cells send signals to the brain and regulate hormone production. The three most important hormones that control the biological rhythm are:

  • Melatonin makes you tired, slows the body functions and lowers activity in favour or earned rest.

  • Cortisol on the other hand is a stress hormone that is produced from about 3 am. It stimulates the metabolism and programs the body for day mode.

  • Serotonin works as a stimulant and motivator. While the cortisol level in the blood drops throughout the day and thereby acts counter-cyclic to the melatonin level, serotonin helps elevate energy levels.