Objectives The aims of the present study thus were (a) to further investigate the association between lithium levels in drinking water and suicide rates by adjusting relevant factors using the so far largest available dataset in Japan, (b) to confirm sex differences, (c) to estimate the effects of long‐term exposure to trace lithium, (d) to investigate the effects of drinking bottled instead of tap water, and (e) to exploratorily investigate which lithium levels may be associated with lower suicide rates. Methods Mean lithium levels in drinking water of all 808 cities and wards (ie, 785 cities of 46 prefectures and 23 wards of Tokyo) in Japan were examined in relation to mean suicide standardized mortality ratios (SMRs) during the 7 years from 2010 to 2016. Multiple regression analyses adjusted for the size of each population were used to investigate the association of lithium levels with suicide SMRs with adjustments for relevant factors. Results The adjusted model showed significant inverse associations of lithium levels with total and male SMRs, but not with female SMRs. Neither the proportion of residents who continued to live in the same city nor the consumption of bottled water changed the association between lithium levels and suicide SMRs. Finally, it was 30 μg/L or more that was associated with lower suicide SMRs. Conclusions The present findings reconfirm the inverse association between lithium levels in drinking water and suicide rates particularly in the male population.

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Bright light exposure (BL) induces neurogenesis in the rat hippocampal dentate gyrus (DG). We had previously conducted a randomized controlled trial (RCT) in which a 4-week period of BL in healthy participants resulted in increased volume of the left DG-head. This study aimed to investigate the effects of BL on the DG in patients with mood disorders. A 4-week RCT was conducted in which patients with mood disorders were randomly assigned to either a BL group (10,000 lx) or dim light exposure group (DL group; 50 lx). All patients underwent clinical assessment and magnetic resonance imaging at baseline and after the intervention. The study registration number is UMIN000019220. Our final sample included 24 patients (BL group, n = 12; DL group, n = 12). A significant effect of time and group was detected in the volumes of the left DG-head (F (1, 62.9) = 11.6, partial η2 = 0.35, p = 0.003) and left DG-total (left DG-total = left DG-head + left DG-body; [F (1, 86.8) = 6.5, partial η2 = 0.23, p = 0.02]). Additionally, the BL group demonstrated a significant increase in the volume of the left DG-head (95% CI: -5.4 to -1.6, d = 1.2, p = 0.002) and left DG-total (95% CI: -6.3 to -1.5, d = 1.06, p = 0.005) as well as a positive correlation between the percentage change in the volume of the left DG-total and the percentage change in the scores of the mood visual analogue scale (r = 0.58, p = 0.04). In conclusion, our study results suggest that compared to DL, BL leads to a significantly greater increase in the left DG volume in patients with mood disorders. This increase in the left DG volume may be associated with mood improvement in the patients.