Background <p>Suicide prevention can be significantly enhanced by time-sensitive surveillance using digital data sources like online message board. To inform more effective suicide prevention strategies, this study analyzes the temporal patterns of posts on a Japanese mental health message board—NHK’s “Facing Suicide” website—that may aid in early risk detection.</p> Methods <p>We analyzed 63,046 posts from Japan’s national broadcaster (NHK) message board (1 Jan, 2008–31 Mar, 2025), stratified by gender and age (≤19, 20s, 30s, ≥40). Generalized additive models were used to model hourly, weekly, and monthly variations, with time of day included as a spline term. Results are presented as incidence rate ratios (IRRs) with 95% confidence intervals (CIs).</p> Results <p>Females contributed 75.5% of posts, and the 20–29 age group was the most active (32.4%). Posting activity consistently peaked around 23:00 across all subgroups. A marked increase was observed among adolescents (≤19 years) in August (males: IRR = 1.30, 95% CI [1.05–1.61]; females: IRR = 1.55, 95% CI [1.43–1.69]), while adults showed decreases in January–February. Weekly patterns varied by subgroup; for instance, males aged 20–29 posted more on Mondays and Tuesdays (IRR = 1.17, 95% CI [1.05–1.30]).</p> Conclusions <p>Online message board activity displays predictable temporal cycles with demographic-specific patterns. These findings provide an essential baseline for real-time monitoring systems to detect deviations that may signal elevated suicide risk. The August peak in adolescent posts aligns with back-to-school distress, and the late-night peaks underscore the need to provide 24-hour support services and implement automated multi-layered online intervention strategies directly within message boards. These insights can guide the development of targeted, time-sensitive suicide prevention strategies.</p>