In our culture, watching television for hours is a daily habit for many people and it's not unusual for older people to settle down for a night of binge watching.
When they finally creak to their feet, they may feel the effect in their joints and muscles but are happily unaware of lasting effects on their brain.
This study, however, has produced a couple of sobering findings that could make long evenings on the couch a touch less comfortable.
First, the loss of cognition was the greatest among those who had the most to lose. Those who entered the study with a strong and healthy verbal memory, suffered the greatest decline over the years.
But there is a positive side to this, says lead author Dr Daisy Fancourt, senior research fellow in the Department of Behavioral Science and Health Behavioral at University College London.
"It's promising because it suggests that this association is not just being found in people already experiencing cognitive decline," she says.
"To find this effect in people who are cognitively healthy at the beginning, suggests that television watching is a modifiable effect and that reducing viewing may be one way of preserving cognition."
Until now, most studies have used sitting in front of the television as a proxy for poor health and sedentariness.
"We wanted to test if it's just the sitting or more than that. It's something about the television itself," Fancourt says.
Another sobering finding is the cognitive effects occur independently of sedentariness.
What surprised her was how often people can watch television before there is an association with cognitive decline.
"We found 70 percent of older adults were watching more than 3.5 hours of television a day. Given we are now seeing an association with poorer cognition 10 years later, maybe we need to raise awareness that there could be other activities that might be better for them
"And we have 20 percent watching over seven hours a day, which is huge."
There were no guidelines for the recommended levels of television viewing for the 3600 older adults in the study, which measured their watching habits in 2008 and 2009, and then rechecked their memory six years later.
The study population was drawn from the Longitudinal English Study of Aging, which has a nationally representative base of older adults.
While the population over 50 is big and very diverse, Fancourt says the study checked if age was a factor and found the effect on cognition was the same above and below 65.
The modeling found was also independent of several other factors, such as health and economic status and people's social activities.
She says two cognitive functions, each controlled by different parts of the brain, were tested.
Only one declined and that was verbal memory. This is the ability to recall a shopping list or remember the name of a school someone mentioned a minute ago.
The other, semantic fluent, was unaffected. It is being able to speak fluent sentences and is often considered to be the result of combining different aspects of executive function within the brain.
So why was verbal memory affected?
One suggestion is that watching television reduces the amount of time people spend on activities that could contribute to cognitive preservation, such as reading, playing board games and engaging with cultural activities.
Another possibility is the alert-but-passive nature of television watching, may create cognitive stress, which could contribute to memory decline.
Laboratory experiments have shown television leads to a more alert but less focused brain. It involves fast-paced changes in images, sounds and action and is the most passive screen-based way of receiving such stimuli.
In addition to cognitive stress, the content of the programs can be stressful. Long term stress has been shown to lead impairment in cognition.
This alert-but-passive response to television has been researched in children with mixed results.
Some studies suggest that it promotes language acquisition and visual motor skills in very young children while others suggest that it is associated with poorer reading, comprehension, maths, language and delay in motor development.
At the other end of the lifespan, there has been a sprinkling of scientific interest in the effects on older people, none of which have had the power of this study.
They have mainly used television viewing as a marker of sedentary behaviour which, in itself, can accelerate cognitive decline in older adults.
One study suggested excessive television watching may increase the risk of developing Alzheimer's while another suggested four hours of daily viewing was linked to poorer short-term memory and fluid intelligence over the next four years.
But television has its benefits too. Apart from providing relaxation and escapism, some research shows watching dramas (rather than documentaries) can increase scores in the "theory of mind" test.
This measures the ability to understand the mental states, desires, beliefs and emotions of others. Then there are education and mind-challenging shows.
This study only considered traditional television viewing. This is relevant to Australia because here, reports to the end of 2017 say free-to-air television remains king.
While behaviours are changing, broadcast television is most popular with Australians aged 60 and over.
One of Australia's leading experts on dementia, Professor Henry Brodaty, says the findings of this study are intriguing, although the mechanism is not clear.
"It's a very well conducted analysis and well-written paper published in a top journal," says Brodaty, from CHeBA (Center for Healthy Brain Ageing), school of psychiatry, UNSW Sydney.
He says the authors worked hard to exclude usual confounders and attempted to exclude reverse causality, which is that those with incipient dementia watched more TV.
They did this by reanalysing and excluding those who developed dementia in the following two years after this follow-up.
"However, this does not completely exclude this possibility as the build-up of pathology to diagnosed dementia can be many more years than that," Brodaty says.
"A further confounder, which they acknowledge, is that it may not be the television watching but that non-watchers may be engaging in more cognitive activities."
He says it is important to stress that association does not mean cause.
Jill Margo is an associate professor associate, University of NSW Sydney,