The Socialization of Birth Timing: A Multi-Decadal Analysis of Day-of-Week and Hourly Patterns in 68,000 Japanese Births (1960–1999)

AbstractHuman birth timing was historically regulated by circadian biological rhythms, with spontaneous deliveries typically occurring during the late night and early morning. However, the increasing medicalization of childbirth raises the hypothesis that social and institutional schedules now exert a dominant influence. This study empirically examines the historical transition from “biological time” to “social time” by analyzing hourly and day-of-week birth distributions among individuals born in Japan between 1960 and 1999. Using a dataset of 135,748 self-reported birth records collected from a commercial astrology web service, and after excluding default values indicating unknown times, the final analytical sample comprised 68,224 individuals. Birth trends were assessed using hour- and weekday-level aggregation, minute-level kernel density estimation, and cross-decade heatmap visualization.Results reveal a pronounced shift toward institutional scheduling. First, weekend births declined markedly, forming a clear weekend gap by the 1990s. Second, the natural dawn peak visible in the 1960s was replaced by a sharp weekday peak between 13:00 and 15:00 in the 1990s, coinciding with typical hospital operating hours; minute-level analysis confirmed that the drop at 12:00 reflects lunch-break scheduling rather than analytical artifact. Finally, heatmaps demonstrate increasing concentration of births within business hours (Mon–Fri, 09:00–17:00). These findings indicate that late-20th-century Japanese birth timing increasingly reflected the logistics of medical practice rather than intrinsic physiological patterns. Beyond documenting this transition, the study demonstrates that large-scale self-reported digital data can replicate epidemiologically robust trends with high temporal resolution, suggesting potential applications in real-time public health monitoring.