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3Â Â Light-emitting diode (LED) technology has revolutionized the lighting industry. The dim- ming and instant-on capabilities of these light sources, along with their high efficiency, have allowed lighting designers to overcome some limitations of previous technologies, particularly in roadway lighting environments. However, concerns related to the health and environmental impacts of LEDs have been raised. An American Medical Association (AMA) report expressed concern regarding the blue content in the LED spectrum and its effect on circadian rhythms in humans (Kraus, 2016). Despite these concerns, no controlled empirical studies have quantified the health effects of any part of the LED spectrum under real-world roadway lighting conditions. The human body has 24-hour circadian rhythms driven by light in the environment. Light also has powerful neuroendocrine and neurobehavioral effects, including regulation of the hor- mone melatonin, which is secreted from the pineal gland. These effects are mediated in part by light stimulation of the eyeâs intrinsically photosensitive retinal ganglion cells (ipRGCs) (Lucas etÂ al., 2014). For healthy circadian and neuroendocrine regulation, humans need exposure to sufficiently bright light during the daytime and darkness at night. In modern industrialized nations, concerns have been raised that nighttime lighting on roadways and in outdoor spaces disrupts normal sleep, circadian rhythms, and neuroendocrine physiology. Such disruptions have been associated with the risk of some cancers, heart disease, and metabolic disorders (Bedrosian, Fonken, & Nelson, 2016). The ipRGCs are most sensitive to blue light, with light in the wavelength range of 459â484 nanometers (nm) having the strongest effects on the circadian system and melatonin regulation (Brainard etÂ al., 2001; Lucas etÂ al., 2014). LEDs typically have higher blue spectral contents than traditional high-intensity discharge (HID) light sources, and exposure to LEDs in the evening has been shown to disturb circadian rhythms, resulting in sleep loss (Cajochen etÂ al., 2011; Chang etÂ al., 2015). On the other hand, some studies have shown that light with a high blue content can increase alertness and enhance cognitive performance in humans (Chellappa etÂ al., 2011; Lehrl etÂ al., 2007). Thus, LED roadway lighting with high blue content might make road users (drivers, pedestrians, and others) more alert, thereby enhancing nighttime traffic safety. To design effec- tive roadway lighting, there is a growing need to understand the relationships between roadway lighting, melatonin suppression, and driver alertness and health. However, these relationships have never been investigated in realistic roadway environments. Results from studies conducted in realistic roadway environments could be readily transferred to the real world, as the condi- tions and dosages encountered would be nearly identical. Thus, such a study would generate recommendations for roadway lighting that are more valid and applicable than those derived from research conducted in indoor laboratories. C H A P T E R Â 1 Introduction