By far, the majority of these studies assesses the responses elicited by exposure to a short duration pulse of light (typically <2 h) against a background of complete darkness or dim light. Interestingly, much of the seminal work characterizing non-image forming responses to light has taken place during the biological night, yet the focus of lighting countermeasures in humans has been on optimizing photic exposure during the day. It has been proposed that the current suboptimal patterns of photic exposure, including insufficient light during the day and excessive light at night, are at least partly responsible for a variety of negative health and safety consequences ( 5, 6). To provide a daily photic temporal cue in lieu of exposure to the solar light dark cycle, a biologically potent light stimulus should be used to signal daytime ( 4), whereas a relatively less potent stimulus (dimmer and depleted in shorter wavelengths) should be utilized during the evening and night. Yet, the timing of photic delivery is also of utmost importance: one major non-image forming function of light is to synchronize internal biological rhythms-orchestrated by a master pacemaker in the suprachiasmatic nucleus (SCN) in the hypothalamus-to environmental light-dark cycles. As current architectural lighting strategies have been developed around the human visual system, most do not fully support biological health and well-being. All of the non-image forming effects of light studied to date appear to be most sensitive to short-wavelength light and at intensities greater than what is required for visual illumination ( 3). In addition to illuminating the environment for vision (i.e., image forming), light also regulates non-image forming processes, such as the circadian timing system, neuroendocrine fluctuations, and acute alerting effects, just to name a few ( 1, 2). Light has a profound influence on the biological health of mammals, including humans. We also provide recommendations for the field of chronobiological research, including minimum requirements for the measurement and reporting of light, standardization of terminology (specifically as it pertains to “dim” light), and ideas for reconsidering old data and designing new studies. Here, we lay out the argument that nLAN exerts potent circadian effects on numerous mammalian species, and that given conservation of anatomy and function, the efficacy of light in this range in humans warrants further investigation. A solid understanding of the impacts of very low light levels is complicated further by the broad use of the somewhat ambiguous term “dim light,” which has been used to describe light levels ranging seven orders of magnitude. It is often assumed that such low levels of light do not produce circadian responses typically associated with brighter light levels. In this review, we discuss the remarkable potency and potential applications of a form of light that is often overlooked in a circadian context: naturalistic levels of dim light at night (nLAN), equivalent to intensities produced by the moon and stars. 4Departments of Psychiatry and Neuroscience, Uniformed Services University of the Health Sciences, Bethesda, MD, United States.3Department of Psychology, University of California, San Diego, San Diego, CA, United States.2Oregon Institute of Occupational Health Sciences, Oregon Health and Science University, Portland, OR, United States.
1Center for Circadian Biology, University of California, San Diego, La Jolla, CA, United States.
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