Background: Recent Italian statistics on suicide distribution by time of day also report data on gender and age of victims, factors which have been shown to influence the seasonal distribution of suicide and which could also affect the influence of biological circadian rhythms on suicidal behaviour. This study aims to identify and evaluate any diurnal variations that may be present in suicide occurrence by age and gender in Italy, considering data from 1994 to 1997.
Methods: The null hypothesis that there are no variations in the distribution of suicides by time of day (or over the three major periods of the day: morning, afternoon, evening/night) was tested with the chi2 goodness-of-fit test and with ANOVA.
Results: A clear diurnal variation in the distribution of suicides over time can be observed for both genders, with a peak in the late morning (08:00-11:00 h), and a subsequent decrease to a trough in the night hours. This trend varies with age for both genders: in particular, the age groups 45-64 and 65+ show a clear suicide peak in the morning (08:00-11:00 h), whereas younger people have a peak number of suicides in the late afternoon (16:00-19:00 h). Adults (25-44 years old) show an intermediate trend, with a less pronounced peak between the morning and early afternoon hours. The observed trend is more marked among males; however, the distribution of suicides by time of day is clearly congruent by age between both genders.
Conclusions: Diurnal variation in suicide occurrence by age group may be affected by factors distributed unevenly across age groups. In particular, age distribution of disorders leading to suicidal ideation, and the sensitivity of biological systems of different age groups to environmental cues may affect each group's risk of suicide. Socio-relational factors are also likely to contribute to diurnal variation in suicide risk by age and gender. Children and adolescents can generally be presumed to be at school during the morning, therefore their opportunity for self-harm is restricted to afternoon hours. The elderly, on the other hand, may find themselves alone in the morning, when family and friends spend more time away from home due to daily work activities.
Limitations: Data are based on time of death and not on presumed time of the suicidal act. For suicides committed by certain methods (e.g., poisoning) there may be a considerable difference between time of act and time of death.
Clinical relevance: The existence of a temporal window in suicide risk implies an improvement in the surveillance of people at risk of suicide and greater attention to chronobiological factors affecting those suffering from mental disorders leading to suicide ideation.