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A new study may have some alarming news for couch potatoes. Men who watch television for 20 hours per week have almost half the sperm count of those who watch very little television or none at all, according to a study published on Tuesday.

The findings also may offer some reassurance to active guys who have heard that exercise might decrease sperm counts, says Jorge Chavarro, an assistant professor of nutrition and epidemiology at the Harvard School of Public Health and a co-author of the study published Monday in the British Journal of Sports Medicine.

"Most of the previous research has focused on endurance athletes, such as marathon runners and professional cyclists," whose bodies may be under unusual stresses, Chavarro says. Biking, in particular, might overheat or injure the scrotum, research suggests.

Exercise or lack thereof played a huge role in the study results. Men who exercised for 15 or more hours weekly at a "moderate to vigorous" rate had a 73-percent higher sperm count than those who exercised less than five hours per week.

None of the sperm levels was so low that the man would have been unable to father a child. Semen quality appears to have declined over several decades, according to studies conducted in several countries.

However, to put the study results into perspective, it was done on a very small group of men. Researchers from the Harvard School of Public Health asked 189 young men who were students at a university in New York to record how many hours they had been spending doing physical activity and watching TV in a typical week. The volunteers, all aged between 18 and 22, were also asked to provide a sperm sample for lab analysis.

The study was funded by the National Institutes of Health and the European Union's Developmental Effects of Environment on Reproduction project.