Ghosts and goblins aren't the only things to be scared of at this time of year. You'll be coming home from work in the dark, and that increases your chance of being robbed.

"When daylight saving time ends, the prime work commute hours in the early evening – when people are walking out to parking garages or heading to public transit – take place after sunset," explained Nicholas Sanders, assistant professor of economics in the Department of Policy Analysis and Management in the College of Human Ecology. "Does this shift affect public safety?" he asked.

Criminals like darkness because it decreases the chance they will be identified by witnesses and makes it easier to slip away, while potential victims are less likely to see them coming. Robberies are less common later at night. For the robber it's a problem of supply and demand: In the early evening there are more people on the streets.

Of course on daylight saving time it may be dark when you're going to work in the morning, but apparently criminals don’t like to get up early.

Sanders and Jennifer Doleac, assistant professor of public policy and economics at the University of Virginia's Frank Batten School of Leadership and Public Policy, analyzed data from the National Incident-Based Reporting System for 2005-08, in 558 jurisdictions covering a total population of 22 to 24 million persons. They focused on felony robbery; in many other crimes, including murder and assault, the people involved already know each other, so time of day and lighting conditions are less likely to be a factor. Most of the jurisdictions examined were in urban areas and in East Coast states.

They found that as daylight saving time (DST) comes to an end in the fall, incidents of reported robbery increase by 7 percent. Most of that is in the hours just during and after sunset, where robberies increase by 27 percent. The results were reported online Oct. 20 in The Review of Economics and Statistics, and will be published in a forthcoming print issue of the journal in a paper titled "Under the Cover of Darkness: How Ambient Light Influences Criminal Activity."

To control for time of year effects, they also looked back to 2007, when Congress extended DST by three weeks in the spring and one week in the fall, hoping to save energy and reduce the nation’s dependence on foreign oil. Ironically, the change actually increased energy consumption, but by comparing days when DST was in effect in 2007-08 with the same days of the year in 2005-06 when it was not, they found a decrease in robberies happened only when DST was in effect, resulting in a social cost savings of at least $59 million annually.

Many factions lobby against extending DST. It messes up airline schedules. Kids go to school sleepy and may be in danger waiting for a bus on a dark morning. On the Monday following the changeover there are more traffic accidents, workplace injuries and heart attacks, blamed on the disruption of circadian rhythms. But most of these costs come from the changeover itself, the researchers said, and are likely small in comparison to the benefits of a substantial drop in violent crime.

"Society could reduce the overall social costs of crime by simply shifting the clock," they concluded.