Human fertility has declined sharply since 1950, and a growing body of evidence suggests that while conventional socioeconomic factors are well-established drivers of the broader demographic transition, they do not fully account for the timing and breadth of this trend. This review examines the Circadian-Light-Hygiene hypothesis, which proposes that daily light exposure is a fundamental regulator of reproductive health. We synthesize findings from photobiology, endocrinology, reproductive medicine, and epidemiology to evaluate how artificial light at night, insufficient daytime light, and irregular light-dark patterns may disrupt the hormonal timing systems that support reproduction. The available evidence indicates that such disruption can alter melatonin signaling, circadian gene regulation, and neuroendocrine rhythms, with downstream effects on ovulation, sperm quality, endometriosis, polycystic ovary syndrome, pregnancy outcomes, and developmental programming. Urbanization, screen use, and shift work appear to amplify these effects, while genetic variation may modify individual susceptibility. Although direct causal evidence in humans remains limited for several endpoints, the convergence of observational, experimental, and translational data supports circadian-light misalignment as a plausible, additional modulator of fertility decline, and a potentially modifiable contributor. Optimizing daily light exposure may therefore represent a low-cost and scalable strategy for improving reproductive health.
