Relationship of Nocturnal Wakefulness to Suicide Risk Across Months and Methods of Suicide

Abstract

Objective: Insomnia is a risk factor for suicide, and the risk of suicide after accounting for population wakefulness is disproportionately highest at night. This study investigated whether this risk varied across months and/or methods of suicide.

Methods: Time, date, method (eg, firearm, poisoning), and demographic information for 35,338 suicides were collected from the National Violent Death Reporting System for the years 2003-2010. Time of fatal injury was grouped into 1-hour bins and compared to the estimated hourly proportion of the population awake from the American Time Use Survey for 2003-2010. Negative binomial modeling then generated hourly incidence risk ratios (IRRs) of suicide. Risks were then aggregated into 4 categories: morning (6:00 am to 11:59 am), afternoon (noon to 5:59 pm), evening (6:00 pm to 11:59 pm), and night (midnight to 5:59 am).

Results: The risk of suicide was higher at night across all months (P < .001) and methods (P < .001). The mean nocturnal IRR across months was 3.18 (SD = 0.314), with the highest IRR in May (3.90) and the lowest in November (2.74). The mean (SD) nocturnal IRR across methods was 3.09 (0.472), with the highest IRR for fire (3.75) and the lowest for drowning (2.44). Additionally, nocturnal risk was elevated within all demographics (all P < .001). However, there were no month-by-time or method-by-time interactions across demographics (all P > .05).

Conclusions: Regardless of month or method, the incidence risk of suicide at night is higher than at any other time of day. Additionally, demographic subgroups did not differentially experience higher risks across months or mechanisms at night.

Figure 1:

The risk of suicide by time of day across months. The incidence risk ratios were much higher at night as compared to the average across all times of day. Risk peaks in May and October, although these increases were not statistically significant. Night: 12AM to 6AM; Morning: 6AM to 12PM; Afternoon: 12PM to 6PM; Evening: 6PM to 12AM. Risk ratios are plotted as mean ± standard error.

Figure 2:

The nocturnal risk of suicide across months by demographic characteristics. Incidence risk ratios were generated within demographic categories (e.g. for males separately from females), so comparisons across demographics were not made. No statistically significant effects for time of day by demographic were noted. Risk ratios are plotted as mean ± standard error.

Figure 3:

The risk of suicide by time of day across methods of suicide. The incidence risk ratios were higher at night as compared to the average risk across all times of day. While vehicles and fires showed an elevated risk, this increase was not statistically significant. Night: 12AM to 6AM; Morning: 6AM to 12PM; Afternoon: 12PM to 6PM; Evening: 6PM to 12AM. Risks are plotted as mean +/− standard error. Risk ratios are plotted as mean ± standard error.

Figure 4:

The nocturnal risk of suicide across suicide methods by demographic characteristics. Incidence risk ratios were generated within demographic categories, so no comparisons across demographics were made. The only statistically significant effect present was the increased nocturnal risk of death by fire in northern states; no other statistical differences were noted. Risk ratios are plotted as mean ± standard error. Categories for which no ratio is present did not have at least 5 suicides at each time of day, and were therefore excluded from analysis.