Exploring lag and duration effect of sunshine in triggering suicide

Abstract

Background

Sunshine is considered to have a beneficial impact on mood. Interestingly, it has been consistently found that the incidence of suicide reaches a peak during early summer.

Methods

In order to explore the pattern of sunshine and suicide risk in a time frame of up to nine days and investigate possible lag and duration parameters of sunshine in the triggering of suicide, Greek daily suicide and solar radiance data were analyzed for a 10-year period using logistic regression models.

Results

The solar radiance during the day before the suicide event was significantly associated with an increased suicide risk (OR = 1.020 per MW/m²). The average solar radiance during the four previous days was also significantly associated with an increased suicide risk (OR = 1.031 per MW/m²). Differences among genders include the longer sunshine exposure needed in males to trigger suicide, compared to females and a lag period of three to four days that was found to lapse in females till the suicide. The increase in suicide risk in June compared to December, attributable to the daily sunshine effect, varies from 52% to 88%, thus explaining the already known suicide monthly seasonality.

Limitations

No individual data on solar radiance exposure, mental disorders, alcohol consumption or suicide method were available.

Conclusion

The effect of sunshine in the triggering of suicide may be mediated through a mechanism with a specific lag and duration effect, during the nine days preceding suicide. We hypothesize that sunshine acts as a natural antidepressant which first improves motivation, then only later improves mood, thereby creating a potential short-term increased risk of suicide initially upon its application.

Introduction

Sunshine is considered to have a beneficial impact on mood and annual total sunshine has been shown to correlate with decreased suicide rates within countries, even after adjusting for confounding factors, such as sociodemographic factors (Nishimura et al., 2004, Preti, 1998, Souetre et al., 1990, Terao et al., 2002). The underlying pathophysiological mechanisms have not been elucidated, but sunshine has been reported to regulate specific hormonal and neurotransmitter levels, such as those of melatonin (Thalen et al., 1997), serotonin (Lambert et al., 2002) and cortisol (Wehr, 1998).

Interestingly, it has been consistently found that the incidence of suicide reaches a peak during spring or early summer (Bazas et al., 1979, Chew and McCleary, 1995, Kevan, 1980, Koskinen et al., 2002, Lambert et al., 2003, Lester and Frank, 1988, Partonen et al., 2004, Petridou et al., 2002, Preti and Miotto, 1998, Souetre et al., 1987). This finding is consistent across countries of both the northern and the southern hemisphere. Moreover, the magnitude of the suicide seasonality pattern, as expressed by the relative risk of committing suicide during the month of maximum versus minimum suicide frequency, is larger in countries with longer sunshine duration during the month of peak suicide incidence (Petridou et al., 2002). A most striking confirmation of this relation was evident in Greece, the country with the longest sunshine duration during the month of peak suicide incidence, which also exhibited the highest relative risk for committing suicide, a 50% increase during June (the month of peak suicide incidence) compared to December (Petridou et al., 2002).

Motivated by this finding, we have focused in this paper on Greek daily suicide and sunshine data, in order to: 1) explore the pattern of sunshine and suicide risk in a time frame of up to nine days before the suicide event, and investigate possible lag and duration parameters of sunshine in the triggering of the suicide, and 2) examine if the indicated pattern of sunshine and suicide risk can explain the monthly suicide variation.