Light-at-night, health, and sex/specific differences: who’s afraid of the dark? A narrative review

For billion of years, alternating light-dark cycles have influenced all biological functions, and living organisms have developed evolved internal circadian clocks capable of adapting to cyclical 24-h fluctuations in the environment. In the modern era, the advent of electric lights allowed light during night hours, and the introduction of less expensive light-emitting diodes (LED) sources favored a widespread increase of light use in at least 80% of the world. However, exposure to artificial light-at-night (ALAN) is associated with a wide spectrum of health problems. We analyzed recently published available literature, including two systematic reviews and one meta-analysis, to investigate the possible existence of sex-specific differences in the effect of ALAN exposure on mental health disorders, sleep problems, and cardiometabolic disorders. We found twelve studies (n = 160,305, 20.8% females) focusing on mental disorders, ten studies (n = 591,554, 44.4% females) focusing on sleep disorders, and ten studies (n = 588,929, 49.8% females) focusing on cardiometabolic disorders. On the one hand, the majority of studies included a valid percentage of females, but only a few studies performed separate analysis by sex. Thus, although the available evidence seems to confirm some negative effects on biological functions under the control of circadian rhythms, no definite conclusions can be drawn on the existence of possible sex-specific differences. The impact of light pollution on human health remains a fascinating area of research, and furher studies are needed to definitely confirm this relationship, especially studies providing robust evidence of possible sex-specific differences.