Suicides are more likely to occur during the dark hours between midnight and sunrise, a new study shows.
The researchers said their findings have important implications for people with chronic insomnia that lasts for at least three months. This sleep disorder affects about 10% of adults. Treating insomnia may help lower suicide risk, the study authors said.
"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 behaviour," lead investigator Michael Perlis, director of the Penn Behavioural Sleep Medicine Programme at the University of Pennsylvania in Philadelphia, said in a news release.
"These results suggest that not only are nightmares and insomnia significant risk factors for suicidal ideation and behaviour, but just being awake at night may in and of itself be a risk factor for suicide," he said.
Suicide 10th leading cause of death
Although previous studies have found that suicides occur more often during the day, the researchers pointed out these studies didn't adjust for the proportion of people awake at certain times of the day.
Using the National Violent Death Reporting System, the researchers were able to analyze the estimated times of suicides. They also obtained information on the proportion of Americans awake at any given hour from the American Time Use Survey.
The times that suicides occurred were divided into one-hour groups, weighted by the proportion of people awake at that time.
Read: Teen suicide: recognise the warning signs
After analyzing information on more than 35 000 suicides, the researchers found the frequency of suicides between midnight and 5:59 a.m. was 3.6 times higher than expected.
The study revealed the mean suicide rate was 10.27% per hour after midnight. It peaked between 2 a.m. and 3 a.m., at 16.27%. Throughout the day, the mean suicide rate per hour was 2.13%.
The study was published in an online supplement of Sleep, and was to be presented at the Associated Professional Sleep Societies annual meeting in Minneapolis. The findings should be considered preliminary until published in a peer-reviewed journal.
More than 38 000 people die by suicide each year, making it the 10th leading cause of death in the United States.
Teen suicide: Why do they want to die?
Jury still out on whether doctors should screen for suicide risk
Doctors often miss suicide warning signs