Practicing Christians tend to watch more television than non-Christians a new study by the Barna Group across the United States has found.

Barna, a California-based faith research company, asked adults 18 and older what TV shows they watch - and if they're watching at all.

"Friends was the sitcom that brought Gen-Xers and Boomers together more than a decade ago.

"In 2014,The Big Bang Theory is the sitcom that brings Millennials and Gen-Xers and Boomers -and even a significant number of Elders -together.

"Thirty percent of all adults regularly watch the show about four uber-nerds navigating life, love and theoretical physics. Gen-Xers and Elders are slightly less likely to tune in than Millennials and Boomers, but more than one-quarter of each generation segment are TBBT watchers," Barna found.

It found practicing Catholics watch an average of 3.5 hours a day and practicing Protestants watch an average of 3.1 hours.

By contrast the survey adherents to faiths other than Christianity watch 2.6 hours of TV per day and those of no faith, which includes self-identified atheists and agnostics, watch 2.7 hours.

'Interestingly, church attendance seems to make little difference in the number of viewing hours.

"Those who attended church within the past week, those who attended within the past month and those who have not attended at all within the past six months all watch an average of 3.2 hours per day," Barna said.

Three quarters of Americans say they watch some TV every day - but the amount of time they spend watching varies greatly.

Barna said, "A decade ago, when anybody with an aerial on the roof, a set of rabbit ears or even a well-placed coat hanger had access to the broadcast airwaves, network TV shows could regularly expect 20 million viewers to tune in to big season finales.

"Now that TV is digital - and delivered not only to the 60-inch screen at home but also to mobile devices any- and everywhere," the survey sought to find "who is watching?"

Adults are almost evenly split between those who watch one to three hours (51 percent) and those who watch four or more hours a day (44 per cent).

A large minority watches five or more hours of television per day (30 per cent) .

On average, women (3.4 hours, median) watch more TV than men (3.0 hours).

And, as a rule, TV watching increases with age. Elders, adults who are 69 and older, watch an average of 4.4 hours a day, while Boomers (born between 1946 and 1964) watch an average of 3.8.

Gen-Xers (born between 1965 and 1983) and Millennials (born 1984 to 2002) watch fewer hours: 2.5 and 2.7 hours, respectively, the survey finds.