Hormonally mediated effects of artificial light at night on behavior and fitness: linking endocrine mechanisms with function

Abstract

Alternation between day and night is a predictable environmental fluctuation that organisms use to time their activities. Since the invention of artificial lighting, this predictability has been disrupted and continues to change in a unidirectional fashion with increasing urbanization. As hormones mediate individual responses to changing environments, endocrine systems might be one of the first systems affected, as well as being the first line of defense to ameliorate any negative health impacts. In this Review, we first highlight how light can influence endocrine function in vertebrates. We then focus on four endocrine axes that might be affected by artificial light at night (ALAN): pineal, reproductive, adrenal and thyroid. Throughout, we highlight key findings, rather than performing an exhaustive review, in order to emphasize knowledge gaps that are hindering progress on proposing impactful and concrete plans to ameliorate the negative effects of ALAN. We discuss these findings with respect to impacts on human and animal health, with a focus on the consequences of anthropogenic modification of the night-time environment for non-human organisms. Lastly, we stress the need for the integration of field and lab experiments as well as the need for long-term integrative eco-physiological studies in the rapidly expanding field of light pollution.

Keywords: ALAN; Glucocorticoid; Hormones; Light pollution; Melatonin; Metabolism; Sleep; Stress; Thyroid; Urban ecology.

Fig. 1.

Sources of artificial light at night (ALAN) in an urbanized environment. Relative illuminance (see Glossary) is given in lux for estimates for human vision. Metal halide lights are usually used in stadiums. There is a general trend for all lighting to switch to LED given the versatile use of colors and cost savings.

Fig. 2.

Different endocrine axes affected by artificial light at night that lead to changes in potential fitness components. Solid lines indicate direct causal pathways and dashed lines indicate indirect pathways. GnIH, gonadotrophin-inhibitory hormone; GnRH, gonadotrophin-releasing hormone; HPA, hypothalamic–pituitary–adrenal; HPG, hypothalamic–pituitary–gonadal; HPT, hypothalamic–pituitary–thyroid.

Fig. 3.

Effects of exposure to ALAN on daily and seasonal concentration of melatonin and testosterone in male urban and forest European blackbirds (Turdus merula). Melatonin (A) and testosterone (B) levels in plasma from an equal number of forest and urban birds exposed to two different treatments: either dark nights (control) or artificial light at night (experimental). Experimental birds were exposed to a constant intensity of artificial light level throughout the night, equal to 0.3 lx, and produced by an incandescent light bulb with a wavelength range between 450 and 950 nm. Control birds were exposed to dark nights. Photoperiod followed the natural variation in daylength in Radolfzell, Germany and was changed on a daily basis. Daytime light intensity was ∼600 lx and was provided by fluorescent white bulbs. Both hormones were analysed on plasma samples obtained from the same individuals at different times. Blood sampling for melatonin was conducted in winter (Jan 25 to 29, 2011). Blood sampling for testosterone was conducted approximately every 3 weeks during the period of Dec 2010 to Aug 2011. Figures adapted from Dominoni et al. (2013a,b).