"The cognitive control decreases after a meal taken in company. This means one becomes more liberal and negligent, taking ones one mistakes less seriously," said psychologist Werner Sommer from Berlin's Humboldt University.
The results of his tests with 32 subjects were published this week in the online journal PLOS ONE.
They compared two lunch situations. One group went to a local restaurant with a friend, were able to choose what to eat, and were given a full hour for their lunch. The others ate alone in the office. Although they were given the same food as the restaurant visitors, they were not given a choice - and only had 20 minutes to eat.
Those who had a long, sociable lunch, felt quieter and less awake than those who had eaten alone. They also performed worse in tests of cognitive control - and were not only less likely to spot their own mistakes, they were also more likely to feel grumpy.
Although the tests were not conducted in a real office environment - and the subjects did not have to work before their lunches, Sommer said he expected the results to be applicable to daily work.
"If you want to work efficiently, a sociable lunch is not so suitable," he said.
But managers who are already dictating memos restricting all workers to lonely lunches at their desks might want to hold back for a moment - the longer, sociable lunches would seem to aid creativity.
This last aspect was not the specific target of the study, but the authors see their work as just the first course of a more extensive menu of work on how eating habits affect people.
Health experts have long tried to stress taking a decent break for lunch, something that is already ignored by 26 percent of workers, according to last year's Stressreport Germany.
A third of those employed in Germany complained in a study conducted by statutory health insurer Techniker Krankenkasse that they were not able to get a healthy lunch at work, while half said they did not have the chance to eat in peace and quiet.