With state and local governments struggling to balance budgets in a still sluggish economy, government employment has fallen by 562,000 jobs since September 2008, a decline of 2.6%. In response, the Obama administration has called for more federal aid—on top of the $250 billion doled out in the 2009 Recovery Act—to help keep state and local government payrolls near prerecession levels.

But supporters of more federal aid implicitly assume that the size of the public sector was optimal before the recession. On the contrary, overstaffing is a serious problem in government, and the best evidence is a simple empirical fact: Government employees don't work as much as private employees. If public-sector employees just worked as many hours as their private counterparts, governments at all levels could save more than $100 billion in annual labor costs.

How do we know that? Are we just dredging up well-worn stereotypes of government employees enjoying shorter work days, prolonged sick leave and extended vacation breaks? In fact, new evidence from a comprehensive and objective data set confirms that the "underworked" government employee is more than a stereotype.

In the past, researchers have measured work time with what are called "contract hours," meaning the time that employers require their employees to work. But many people routinely take work home with them, or skip lunch breaks, or pass up vacation days, or go to the office on weekends. Others may regularly come to the office late and duck out early. Little of this variation is captured by contract hours.

Alternatively, researchers have used surveys that ask individuals how many hours they usually work each week. But answers are susceptible to exaggeration and subjectivity regarding what each respondent defines as "work."

To address these problems, we turned to the American Time Use Survey, which the Bureau of Labor Statistics administers to a large and representative sample of American households each year. Interviewers construct a comprehensive "time diary" for each respondent that describes activities that occurred during the entire 24-hour day before the interview. Survey administrators then place each respondent's raw answers into a detailed set of activity categories, one of which is work for a primary job.

The time-use survey's data on work time are far more comprehensive and objective than any other available data source. The survey doesn't undercount working at home versus working at an office, or working evenings rather than working regular business hours. If, for example, an individual was working at 2 a.m. on the weekend, the American Time Use Survey will account for it.

The data allow us to analyze both the number of hours individuals work during a typical workweek and the total number of hours they work during the year. Thus, we can capture differences in both weekly work hours and the amount of time off that employees enjoy throughout the year.

What we found was that during a typical workweek, private-sector employees work about 41.4 hours. Federal workers, by contrast, put in 38.7 hours, and state and local government employees work 38.1 hours. In a calendar year, private-sector employees work the equivalent of 3.8 more 40-hour workweeks than federal employees and 4.7 more weeks than state and local government workers. Put another way, private employees spend around an extra month working each year compared with public employees. If the public sector worked that additional month, governments could theoretically save around $130 billion in annual labor costs without reducing services.

We've excluded teachers from the full-year comparison because of their naturally shorter work year. But could public-private differences in work time be due to other occupational differences between the sectors? Large differences in work hours actually persist even when comparing workers with similar jobs and similar skills in each sector.

Based on the most detailed and objective data set available, the private sector really does work more than the public sector. This fact may hold different lessons for different people, but our own take is simple: Before we ask private-sector employees to work more to support government, government itself should work as much as the private sector.

Mr. Biggs is a resident scholar at the American Enterprise Institute. Mr. Richwine is a senior policy analyst at the Heritage Foundation.