Non-visual effects of light: implications for design

Abstract

Light is not just for vision anymore! Illumination design should consider the needs of the visual and non-visual systems. Discussed are the lighting characteristics impacting these systems and the implications for designing light for architecture.Circadian system, circadian light, spectrum, core body temperature, suprachiasmatic nuclei THE CIRCADIAN SYSTEMLight entering the human eye is transmitted to several other parts in the brain, not just those associated with vision. The world rotates around its axis and, as a result, all creatures on earth experience 24-hour cycles of light and dark 1 . Living organisms have adapted to this cycle by developing biological rhythms that repeat at approximately 24 hour intervals, called circadian rhythms (Latin: circa, about; dies, day).The circadian timing system is composed of three elements: the pacemaker, the input pathways and the output pathways. Circadian rhythms are generated by an internal master clock and are constantly aligned with the environment by zeitgebers (time givers), which are external to the body. The suprachiasmatic nuclei (SCN), known to be the master clock in humans, have an average natural period slightly greater than 24 hours. Environmental cues can reset and synchronize the SCN daily, ensuring that humans' behavioral and physiological rhythms are in synchrony with the daily rhythms found within their environment. Although the circadian system shares receptors and neurons in the retina with the visual system, the retinal ganglion cells exiting the eye for the visual centers are different than those exiting the eye for the circadian system. In 2002, the intrinsically photosensitive retinal ganglion cell (ipRGCs), a novel photoreceptor type in the retina was discovered. 2 The ipRGCs are central to an important "non-visual" response to light by the retina, most notably the regulation of circadian rhythms. It is now well accepted that all three photoreceptors in the retina (rods, cones and ipRGCs) participate in how the retina converts light signal into neural signal for the circadian system.Output pathways include overt rhythms of hormone production and behavior. The most commonly measured output rhythms include hormone production, core body temperature, sleep/wake cycles, and rest/activity rhythms. Melatonin is a hormone produced by the pineal gland at night and under conditions of darkness. Because melatonin production is associated with nighttime, it is called the hormone of darkness. Melatonin gives the body time cues, and it is used as a marker of the circadian clock. Melatonin can be measured in plasma, saliva or urine. Sufficient light at night will acutely cease melatonin production and, depending on the timing of exposure, light can phase shift the timing of melatonin production. Core body temperature is also used as a marker of the circadian clock. It reaches a peak late in the afternoon/early evening and a trough late at night/early in the morning. Core body temperature has an inverse relationship with melatonin. The sleep/wake cycle and rest/...