Risk for Suicide and Homicide Peaks at Night: Findings From the National Violent Death Reporting System, 35 States, 2003-2017

Objective: The Mind after Midnight hypothesis proposes that nocturnal wakefulness increases the risk for dysregulated behaviors. Prior studies highlight a greater risk for suicide at night after adjusting for population wakefulness. How this risk varies hour to hour, differs across subgroups, or applies to other behaviors is unknown. Methods: Data on 78,647 suicides and 50,526 homicides from the National Violent Death Reporting System were combined with population wakefulness data for 2003-2017 from the American Time Use Survey. Hourly incident risk ratios (IRRs) were estimated after adjusting for population wakefulness. Two-way analysis of variances identified significant time-by-subgroup interactions that were quantified in post hoc analyses. Results: Suicide counts peaked at 12:00 PM, while homicide counts peaked at 10:00- 11:00 PM. Adjusting for demographics and population wakefulness revealed a 5-fold greater risk for suicide at 3:00 AM (aIRR: 5.20 [4.74-5.70]) and an 8-fold greater risk for homicide at 2:00 AM (aIRR: 8.04 [6.35-10.2]). Hourly risk for suicide varied by age, ethnicity, blood alcohol level, and current partner conflict. Hourly risk for homicide varied by sex and blood alcohol level. Conclusions: Risk for suicide and homicide is greater at night than expected based on the number of people awake at that time. Nighttime risk was greater among young adults and those intoxicated with alcohol, but not among those with a history of suicidal ideation or attempts. Further research should evaluate mechanisms of risk and confirm these findings at an individual level.