Birth rates may rise now that football season is over and the road to the Final Four has not yet opened. Men who watch more than 20 hours per week of TV have a sperm count almost half that of men who watch little TV. And men who get 15 or more hours per week of moderate to vigorous exercise have sperm counts that are 73 percent higher than those of men who do not get much exercise.

"We wanted to look at this [question of lifestyle and semen quality] because in the past few decades there has been evidence that semen quality has been declining in Western countries, and there has been a lot of [speculation] about why this is happening," Audrey Gaskins, an author of the paper and doctoral student in the department of nutrition and epidemiology at Harvard’s School of Public Health, told TheDoctor.

The researchers saw the increasingly sedentary lifestyle characteristic of Western countries, often a result of too much screen time, as a possible contributing factor.

Men who watch more than 20 hours per week of TV have a sperm count almost half that of men who watch little TV.

They analyzed the semen of 189 men between the ages of 18 to 22 to see if the hours that they spent per week watching TV (an indicator of a sedentary lifestyle) and the hours they spent per week engaging in moderate to vigorous physical activity were associated with semen quality.

The men were asked about the quantity and intensity of their exercise and how much time they'd spent watching television, DVDs, or videos during the preceding three months. Participants were also asked about other factors that might affect sperm quality — including medical or reproductive health problems, diet, stress levels, and smoking.

It is hard to tell from this study alone why semen quality has deteriorated because the researchers only looked at one marker of sedentary behavior, the amount of time spent watching TV per week, says Gaskins. "But based on our biological hypothesis, we believe that sedentary behavior, rather than actually watching TV, negatively affects semen quality."

Poor semen quality is likely the result of a number of factors. The increase in BMI and body weight over the last two decades could play a role. But then, these are connected to exercise — or the lack of it — as well. Chemicals in the environment and in the food we eat may also affect sperm counts. The type of exercise may also make a difference.

Sperm counts are, at best, only an approximate indicator of male fertility. The gold standard is fathering a child. So the researchers are now looking at a group of men recruited from a fertility program at Massachusetts General Hospital. "What is nice about this [group] is that we can look into the effect of lifestyle and other factors on semen quality, but then take it one step further and look to see if it has an effect on the more clinical outcomes of…pregnancy and live birth," says Gaskins.

The message seems to be that fit men have fitter sperm. The study is published online in the British Journal of Sports Medicine.