As the existence and role of melanopsin and the intrinsically photosensitive retinal ganglion cells (ipRGC) in the inner retina have become clearer, so has the realization that current methods of light measurement are incomplete ( 16). During that time, inexpensive lux meters were used because of their existing role in lighting and photography. Until 2000, the majority of photometric studies quantified light stimuli in terms of photopic illuminance (lux) ( 16). All have important roles, with advantages and limitations. Studies considering the potential impact of light exposure at night have employed a variety of methodologies, including animal studies ( 14), laboratory-based controlled-environment studies ( 3– 6), and epidemiological studies ( 13, 15). Furthermore, these same LE devices allow access to the Internet, social media, and games as well as reading, with evidence that multi-tasking is becoming the norm rather than the exception ( 13). This is not an insignificant issue with over a quarter of the US population reading e-books in 2014 ( 12).
In comparison, the same book read in electronic format will provide a very different light signal with biological effects. The development of LE devices means that for many people, a “book at bedtime” is now often an “e-book.” Traditional paper books with dim incandescent bedside lighting reflected off the pages of the book expose the readers to a low-intensity tungsten light with a yellow–red spectrum that has little impact on sleep. Many older LE devices have been shown to have peaks specifically in these short wavelengths ( 3). Short-wavelength-enriched light (blue-enriched) is likely to cause the most disruption, as it most effectively suppresses melatonin ( 10) and increases alertness ( 11). Importantly, not all colors of light have the same effect. While light during the daytime can beneficially enhance alertness, performance, and mood ( 8), in the evening it can suppress the production of melatonin, increase alertness, and delay sleep onset ( 9). The hormone melatonin (“the hormone of darkness”) is produced at night, with the duration of secretion mirroring the dark period, and its production is associated with sleep ( 7). In particular, the light/dark cycle is critical in synchronizing the circadian (daily) clock to the 24 h day. The role of light and its influence on many aspects of our physiology, behavior and well-being is increasingly well understood ( 4– 6). When modern LE devices are used in the evening before bedtime all these factors combine to produce a “perfect storm,” which can adversely affect sleep. The brightness, timing, color, pattern, and the duration of light exposure all influence important physiological body rhythms ( 4– 6). A growing body of evidence suggests that the use of light-emitting (LE) devices in the evening may adversely affect sleep quality and timing, daytime performance, health, and safety ( 1– 3).
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