A comprehensive new look at how mothers fill their hours suggests it’s time to revise the old saying that: “A man may work from sun to sun, but a woman’s work is never done.” Perhaps with something like: “24/7, there’s something to see. Make your own bed, I’m watching TV.”

Compared with mothers in 1965 with children between ages five and 18 at home, contemporary mothers spend on average 11 fewer hours per week in physical activity – including housework, meal preparation, child care, laundry and exercise. They are, however, logging seven more hours per week in sedentary activity, such as watching TV, surfing the internet or driving.

Mothers of children under five have shed about 14 hours of physical-activity time weekly, but have increased their weekly log of sedentary time by six hours, on average.

The new research, published  in the Mayo Clinic Proceedings, looked at data collected in the American Heritage Time Use Study, which has detailed the time Americans engaged in paid and unpaid work from 1965 to 2010. The researchers, led by University of South Carolina public health professor Edward Archer, looked only at how mothers allocated their time outside of paid work hours.

The dramatic reallocation of mothers’ time has consequences for the women themselves. The resulting reduction in energy burned up in physical activity averages 177 calories more per week for women with older children at home, and 225 calories for those with pre-schoolers.

And because mothers’ patterns of activity, consumption and weight gain profoundly influence those of their children from gestation onwards, this 45-year shift from exertion to inertia has likely played an outsized role in the epidemic of overweight and obesity.

“A mother’s behaviours affect the environments to which her progeny are exposed,” Archer said. As a result, changes in mothers’ habits, weight and propensity to diseases are likely being transmitted across the generations.

“With each passing generation, mothers have become increasingly physically inactive, sedentary and obese, thereby potentially predisposing children to an increased risk of inactivity, adiposity and chronic non-communicable diseases. Given that physical activity is a prerequisite for health, it is not surprising that inactivity is now a leading cause of death and disease in developed nations,” Archer said.

The authors gauged the energy expenditure of mothers by mining time-use sheets for detailed break-outs of hours spent daily in distinct activities. They assigned different metabolic equivalents – and thus, calorie expenditure – to doing laundry and cleaning up after dinner, for instance, than leisure-time physical activity.

In 1965, mothers with older children at home spent, on average, 32 hours in “activity time” – a mix of exercise for pleasure and active unpaid work such as child care and housework – and 18 hours in sedentary activities. By 2010, activity time fell to 21 hours per week, while sedentary time rose to 25 hours per week. Those with children under five at home went from 44 hours of activity time per week in 1965 to an average of 30 hours in 2010. Sedentary time, however, went from an average of 17 hours per week to 23 hours.

Most of those changes came in the 1960s, ’70s and ’80s, with the allocation of time to sedentary and physical activity having largely stabilised since the 1990s.

The expansion of time spent on the couch “may be the greatest public health crisis facing the world today,” Archer and his co-authors wrote. Los Angeles Times