Many people enjoy putting their feet up and getting stuck into a box set at the end of a stressful day.
A night in front of the TV may be commonplace, however, US scientists have warned even "sometimes" watching the box could be linked to a decline in cognitive function.
The scientists carried out three studies with 12,900 middle-aged participants between them.
Results reveal those who claimed to watch TV sometimes, often or very often had a higher rate of cognitive decline, as well as a greater reduction to their brain's grey matter volume, which controls all the vital organ's functions.
People may indulge in a box set when they could be exercising, with a sedentary lifestyle increasingly being linked to poor brain health. Nevertheless, TV watching was associated with cognitive decline even after the scientists accounted for the participants' activity levels.
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Unlike some other sedentary hobbies, like reading or playing a board game, TV watching is considered "non-stimulating", which may make people less sharp over time.
One expert allows that TV watching could be up amid the pandemic, but stresses the risk factor is "very easy to modify".
The results were presented at the American Heart Association's (AHA) Epidemiology, Prevention, Lifestyle & Cardiometabolic Health conference and are yet to appear in a peer-reviewed journal.
"While studies have shown the benefits of exercise to support brain health, less is known about the potential consequences of prolonged sedentary behaviour, such as television viewing, on brain structure and function," said Dr Kelley Pettee Gabriel, lead author of one of the studies, from the University of Alabama at Birmingham.
"This is important to look at because other studies have shown physical activity and sedentary behaviours may have different effects on health and disease.
"Engaging in healthy behaviours during midlife, between ages 45 to 64 years in the context of our study, may be important factors to support a healthy brain later in life."
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Cognitive decline is defined as a reduced ability to remember, think, reason, communicate or solve problems.
While this naturally comes about with age, more severe cognitive decline can be a sign of dementia, which affects 850,000 people in the UK alone.
"There are no medications available to cure or stop dementia, however, a recent report showed nearly 40% of worldwide dementia diagnoses may be prevented or delayed by modifying 12 risk factors including exercise," said Dr Priya Palta, lead author of another of the studies, from the Columbia University Vagelos College of Physicians and Surgeons in New York.
To better understand the impact of a sedentary lifestyle, the US scientists asked the three studies' participants how often they watched TV: never or seldom, sometimes, often or very often.
In the first study, the Columbia scientists found adults – average age 59 – who watched TV "sometimes" or "often or very often" experienced a 6.9% greater decline in cognitive function over the next 15 years than those in the "never or seldom" group.
Similarly, the Alabama scientists have revealed the participants in the "sometimes" or "often or very often" TV-watching groups had a lower grey matter volume a decade later than those who "never or seldom" took up the pastime.
A decline in grey matter volume "indicates greater brain atrophy [a loss of nerve cells in the vital organ and the connections between them] or deterioration", according to the scientists.
Finally, a team from the Johns Hopkins Bloomberg School of Public Health in Baltimore analysed 599 people aged 30, who were followed-up 20 years later.
They also linked greater TV watching in early to mid-life to a lower grey matter volume down the line.
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Across all three studies, the results remained the same after the scientists adjusted for the participants' exercise levels, suggesting another factor may be at play.
"In the context of cognitive and brain health, not all sedentary behaviours are equal," said Dr Ryan Dougherty, lead author of the John Hopkins study.
"Non-stimulating sedentary activities such as television viewing are linked to greater risk of developing cognitive impairment, whereas cognitively stimulating sedentary activities (e.g. reading, computer and board games) are associated with maintained cognition and reduced likelihood of dementia.
"Sedentary behaviour may impart a unique risk with respect to brain and cognitive health.
"This is an important finding since it is now well accepted the neurobiology of dementia, including brain atrophy, begins during midlife.
"That's a period where modifiable behaviours such as excessive television viewing can be targeted and reduced to promote healthy brain ageing."
The scientists have stressed the participants' self-reported their TV watching, which is also just one type of sedentary behaviour.
Nevertheless, Dr Mitchell Elkind – president of the AHA – added: "This research is very timely and important in the midst of the COVID-19 [the disease caused by the coronavirus] pandemic because we know people are spending more time engaging in sedentary behaviours, such as watching television while being in quarantine.
"Television viewing is just one type of sedentary behaviour, yet it's very easy to modify and could make a big difference in maintaining and improving brain health."
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