The Non-Visual System

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Before electric lighting was available human activity or rest was to a large extent defined by the presence or absence of daylight. The human circadian system, as the platform from which humans operate to perform all activities, is trained to depend on the exposure of daylight. However there are different ways for light to affect the non-visual system. Visible light entrains the circadian system of the body, it synchronizes behavioral and physiological rhythms based on the light-dark circle of a 24h solar day. It influences the secretion of the hormones like melatonin (responsible for slowing down body functions and lowering activity levels) and serotonin (mood enhancing and increasing performance) and their delivery with the bloodstream to different parts of the body in order to control body functions. And it influences performance, fatigue, cognitive, behavioral and emotional effects. Ultraviolet light affects the production of vitamin D, physiological effects, reddening of the skin and chemical changes. And infrared light causes heating of the skin, affects physical and mental performance, as well as cold-, heat- and pain sensation.

Melatonin secretion.jpg

The actual wavelengths in daylight vary over the day, with meteorological conditions, with latitude and with season. This variability in spectral content and the fact that at all times it is a continuous spectrum with elements in all parts of the visible wavelength range, separates daylight from electric light sources. Thus daylight is better matched to the spectral sensitivity of the circadian system. In the retina of the human eye, different photoreceptors are responsible for the response of the human visual system to visual stimuli. There are two types of photoreceptor cells. Cones are responsible for color vision under normal daylight conditions during the day. They can be differed in three types, each sensitive to different wavelengths ranges, either red, green or blue. Under low light levels at night, as well as for the detection of motion, rods which are about 1000 times more sensitive to light than cones, provide the main visual function. In twilight situations in the morning or the evening, both rods and cones contribute to the image seen by the brain.

Section Key Resources
  • P. Boyce, C. Hunter, O. Howlett, (2003): The Benefits of Daylight Through Windows, Lighting Research Center, Rensselaer Polytechnic Institute, USA,, accessed September 2010
  • Mahnke, F.H. (1996): Colour, Environment and Human Response, Van Nostrand Reinhold, New York, USA
  • Boubekri, M. (2008): Daylighting, Architecture and Health, Building design strategies, Elsevier Architectural Press, Oxford, UK
  • Illuminating Engineering Society of North America (2000): The IESNA Lighting Handbook, Reference and Application, Ninth edition, IESNA, New York, USA
  • Fördergemeinschaft Gutes Licht, Lichtwissen 19, Impact of Light on Human Beings,, Frankfurt, Germany

Lead Author(s): Aris Tsangrassoulis

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