Weak associations between the daily number of suicide cases and amount of daily sunlight

- Suicide cases have an uneven distribution within the calendar year.
- Light from either natural or artificial sources has significant impacts on mood.
- Association between daily sunshine duration and suicide risk is ambiguous.
- We demonstrated a highly significant positive association between the two variables.

Several environmental factors with periodic changes in intensity during the calendar year have been put forward to explain the increase in suicide frequency during spring and summer. In the current study we investigated the effect of averaged daily sunshine duration of periods with different lengths and 'lags' (i.e. the number of days between the last day of the period for which the averaged sunshine duration was calculated and the day of suicide) on suicide risk.We obtained data on daily numbers of suicide cases and daily sunshine duration in Hungary from 1979 to 2013. In order to remove the seasonal components from the two time series (i.e. numbers of suicide and sunshine hours) we used the differencing method. Pearson correlations (n = 22,950) were calculated to reveal associations between sunshine duration and suicide risk.The final sample consisted of 122,116 suicide cases. Regarding the entire investigated period, after differencing, sunshine duration and number of suicides on the same days showed a distinctly weak, but highly significant positive correlation in the total sample (r = 0.067; p = 1.17*10− 13). Positive significant correlations (p ˂ 0.0001) between suicide risk on the index day and averaged sunshine duration in the previous days (up to 11 days) were also found in the total sample.Our results from a large sample strongly support the hypothesis that sunshine has a prompt, but very weak increasing effect on the risk of suicide (especially violent cases among males). The main limitation is that possible confounding factors were not controlled for.