The participants, each aged 18 to 30 at the start of the project, had their physical activity and television viewing habits recorded during repeated visits.
At the end of the study, they were assessed in a series of tests to measure their thinking skills – including processing speed, executive function and verbal memory.
The researchers, whose results were published in the journal JAMA Psychiatry, found that there was a strong link between the number of hours spent in front of the television and poor brain power.
The impact seemed to be even greater among those who watched lots of TV and also carried out little exercise.
People who watched more than three hours of TV a day, and were physically inactive, were twice as likely to fall into the ‘poor’ bracket of cognitive performance.
The scientists warned that the findings are of deep concern – particularly as so many young people today spend hours each day staring at a computer or TV screen, and doing little exercise.
The team, led by Tina Hoang, of the Northern California Institute for Research and Education, wrote: ‘We found that low levels of physical activity and high levels of television viewing during young to mid-adulthood were associated with worse cognitive performance in midlife.
‘In particular, these behaviours were associated with slower processing speed and worse executive function but not with verbal memory.
‘Participants with the least active patterns of behaviour – both low physical activity and high television viewing time – were the most likely to have poor cognitive function.’
The study only looked at the statistical link between television viewing and brain power – and the researchers did not investigate why the two might be connected.
But previous studies have found that being sedentary for too long increases blood pressure and blood sugar levels, which might play a role.
The authors wrote: ‘Television viewing may also be associated with different cognitive and social patterns, depression and poor dietary patterns.’
Dr Andrew Przybylski, a psychologist at Oxford University, said that the research fell down on the fact that participants’ cognitive power was not tested at the beginning of the study – only at the end.’
Experts are increasingly concerned about the impact of ‘screentime’ on the brain – particularly in the social media era when children spend hours glued to computers and smartphones.
A recent study by University College London found that teenagers who use websites such as Facebook within 30 minutes of bedtime tend to perform worse in class.
The researchers found this may be because it over-stimulates the brain, making it difficult to sleep afterwards.
Teenagers are then less likely to get the recommended ten hours’ sleep they need to function well at school the next day.
In a survey of 16 to 19-year-olds, 70 per cent said they use social media before bed – and they achieved on average 20 per cent worse grades in GCSE and A-level exams than those who did not