The Strange Tie Between Daylight Time and Assault Rates

November 5, 2017

You’d think more sleep means less crankiness, but apparently not, according to a new study.

The Monday after Daylight Saving Time ends in the fall is more dangerous than other Mondays, according to a recent study published in Journal of Experimental Criminology. Aggregated police records from across the country show a slight uptick in assaults the Monday after we gain an hour in the fall, followed by a mirror decrease when we lose an hour in the spring.

The surprising results conflict with commonly held beliefs on sleep and mood.

“We were expecting the opposite of what we found,” Rebecca Umbach, the study’s co- author and a criminology doctoral student at the University of Pennsylvania, said. “In the spring, people should be cranky and angry and act accordingly.”

Instead, she found that the lack of sleep muted aggressive behavior and a 3% uptick in assaults, which range from punching someone to attacks involving a weapon. “Our brains are just a little bit foggier than usual,” she observed. “We’re moving around in a daze and might not rise to the bait of acting aggressively.”

The study showed a 3% increase in assaults the Monday after daylight savings ended followed by a 3% decrease next spring. Umbach and her team determined the assault rates using a combination of police data submitted to a national database and numbers from individual departments in major cities like New York and Chicago. They then looked at assault rates the Monday immediately after the time change then compared it to assault rates the following two weeks. Since another study showed that more daylight means less crime, Umbach only looked at assaults after clocks were changed twice a year.

This new study suggests that even a single hour difference in sleep can lead to reverberating consequences outside the amount of winks you get. But sleep research remains confused about what the effects of deprivation exactly are. Some research has found that a poor night’s rest can limit self control, but others have come to the conclusion that sleep deprivation lowers testosterones and curb aggressive behavior, regardless of the subject’s bad mood.

But Umbach thinks that these aren’t conflicting conclusions, but rather complementary ones that explain how sluggishness contributes to a decrease in assaults the Monday after daylight savings time.

However, it’s harder to determine what causes the uptick in violence in the fall. Umbach noticed that while assault rates returned to usual rates in the spring after Daylight Saving Time kicked in, they stayed about as high after it ended in the fall. The jump in assault rates stayed consistent for the two weeks after the return to standard time making it difficult to isolate the effects of the time change. Umbach cited changes in behavior from erratic weather like sudden snowstorms as a possible explanation, although she advises further research.

James Gangwisch, a sleep researcher and assistant professor at Columbia, offered a possible explanation for the assault rates come fall: The change in sleep schedule – even by an hour – can cause irritability, distraction and pessimistic behavior.

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