IN MOST Muslim countries life slows down during Ramadan, the holy month in which the faithful fast during daylight hours. Many people nap during the day and feast at night. Working hours are reduced. Businesses open later and close earlier. In general, less gets done.

There is much anecdotal evidence that Ramadan, which ends on July 5th, has a negative economic effect on Muslim countries. But until recently, no one had properly studied the question. “There is a sensitivity when it comes to Islam,” says Rumy Hasan of the University of Sussex in Britain. But the holy month’s features actually make it easier to study.

The Islamic calendar is lunar, so Ramadan rotates through the seasons. In Egypt, for example, the holy month now falls during the long days of summer. But in 15 years, it will occur in winter, when the days—and, therefore, the fasts—are shorter. The opposite is true for Muslims in southern locales. This cycle, unrelated to other factors that might affect the economy, “presents a kind of naturally occurring experiment”, wrote Filipe R. Campante and David H. Yanagizawa-Drott of Harvard University in the New York Times. “Religious practice is precisely varied and everything else is left in place.”

In a study published last year in the Quarterly Journal of Economics, Messrs Campante and Yanagizawa-Drott looked at data from nearly every country over the past 60 years and found that longer fasting times had a deleterious effect on economic growth in predominantly Muslim countries—not just during Ramadan, but throughout the year. If, say, the average Ramadan fast were to increase from 12 hours to 13 hours, output growth in that country for the year would decline by about 0.7 of a percentage point, they found. “It is a robust negative relationship,” says Mr Yanagizawa-Drott.

Other research suggests that Muslims are less productive during Ramadan. A study by Heather Schofield of the University of Pennsylvania showed that fasting by Indian agriculture workers led to a 20-40% drop in productivity when the holy month fell in the planting or harvesting season. Office workers are said to put off meetings and decisions until after Ramadan, during which trading activity tends to decline on stockmarkets in the Middle East.

But Messrs Campante and Yanagizawa-Drott found that the most important reason for lower growth was that Muslims choose to work fewer hours. They are seemingly no less productive in years when fasts are longer. Surveys indicate that during those years they value work less and religion and leisure more. “You could say it is a healthy shift in attitudes,” says Mr Yanagizawa-Drott. Indeed, fasting Muslims report being happier in years when the days are longer, despite the economic costs.

Many merchants do better around Ramadan thanks to an increase in consumption. In this way, it is like holidays everywhere. But making more thorough comparisons is difficult because it is hard to isolate the economic effect of, say, Christmas. Ramadan’s variability gives researchers something to chew on, even as their subjects go without.