A new study by researchers at Penn Medicine is the first to reveal that suicides are far more likely to occur between midnight and 4 a.m. than during the daytime or evening. The research abstract was published recently in an online supplement of the journal Sleep and will be presented at SLEEP 2014, the 28th annual meeting of the Associated Professional Sleep Societies LLC.

“This appears to be the first data to suggest that circadian factors may contribute to suicidality and help explain why insomnia is also a risk factor for suicidal ideation and behavior,” said principal investigator Michael Perlis, PhD, associate professor in the Department of Psychiatry and Director of the Penn Behavioral Sleep Medicine Program at Penn. “These results suggest that not only are nightmares and insomnia significant risk factors for suicidal ideation and behavior, but just being awake at night may in and of itself be a risk factor for suicide,” he said.

For the study, the researchers analyzed data from National Violent Death Reporting System, which provided data for the estimated time of fatal injury, and the American Time Use Survey, which provided an hourly proportion of the American population that is awake. A total of 35,332 suicides were included in the analysis.

Results show that the weighted, scaled mean suicide rate per hour was 10.27 percent after midnight, peaking at 16.27 percent between 2 a.m. and 2:59 a.m.  In contrast, the mean suicide rate per hour was 2.13 percent between 6 a.m. and 11:59 p.m.  When six-hour time blocks were examined, the observed frequency of suicide between midnight and 5:59 a.m. was 3.6 times higher than expected.

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