The stresses and strains of caring for someone for 50 hours or more a week leads to “accelerated cognitive decline” in middle-aged and older people, research shows.

However, providing care for only five to nine hours a week has the opposite effect, boosting brain health so much that the benefits last until older age.

Carers UK called the findings “extremely worrying” and said they highlight how long hours spent providing care raises the risk of social isolation and burnout.

Dr Baowen Xue, an academic at University College London and the lead author of the paper, said: “Our study shows that the caring responsibilities many people take on in later life can be a double-edged sword.

“On the one hand, lighter caring responsibilities can be good for you by providing mental stimulation from interacting with loved ones or others you’re helping and a sense of purpose and usefulness.

“But being overloaded with caring tasks has exactly the opposite effect and can accelerate people’s mental decline in terms of not being as mentally sharp or quick-thinking as they used to be.”

Researchers compared the cognitive health of 2,765 carers aged 50 or over with that of 2,765 non-carers the same age who are part of the English Longitudinal Study on Ageing. They focused in particular on their executive function – such as their ability to make decisions and juggle competing tasks – and their memory. Participants were 60 years old on average and women comprised 56% of the group.

The paper, published in the journal Age and Ageing, said: “Taking on manageable levels of caregiving may provide cognitively stimulating activities and coordinating care that helps maintain executive function in later life.

“Providing a few hours of support outside the household may help caregivers maintain their cognitive health as they age.”

In contrast, though, “carers providing 50+ hours of care a week exhibited accelerated cognitive decline, indicating that the cognitive stimulation associated with caring is overshadowed by the demands of high-intensity care”, the researchers wrote.

People who care for such long hours are usually full-time carers who as a result have little opportunity to work or enjoy a social life, they point out. “The intensity of such care may lead to feelings of loneliness and disrupt sleep, further compounding its negative effects on cognition.”

The UK’s last census in 2021 found that 5.8 million people provide unpaid care and that 1.7 million of them do so for at least 50 hours a week.

Just over half of all carers have increased the amount of time they provide care, according to Carers UK research last year.

“Caring has a profound impact on carers’ health and wellbeing,” it added. The charity’s research found that 74% of carers feel stressed or anxious, 40% feel depressed and 35% say their mental health is bad or very bad.

Helen Walker, the Carers UK chief executive, said: “These findings are extremely worrying, showing that many hours of caring could contribute to cognitive decline.” The government, local councils and the NHS need to do more to give family carers more support, she added.

The proportion of adults providing care for more than 35 hours a week rose by 71% between 2003-04 and 2023-24, according to Joseph Rowntree Foundation research last year for the IPPR thinktank.

The UCL researchers also found that caring for someone within the carer’s household led to a quicker decline in cognitive function than if caring for someone outside the household.

The paper’s authors urged the government to do more to help “intensive” carers – those with high caring workloads – through better access to funded formal and replacement care.

“By 2040, around 20% of adults in England will be living with major illnesses. With the NHS struggling to cope and social care in crisis, much of this growing demand for care will fall on family members and friends who step in as unpaid carers.

“Our findings show that this shift has profound implications: carers’ wellbeing is often overlooked and there is a real danger that many people overburdened with caring responsibilities will suffer the consequences.”

Caroline Abrahams, the charity director at Age UK, said: “Anyone who cares intensively is likely to struggle to fit in the time for enough rest, sleep and time away doing things they enjoy. These are essential human needs which, when met, set you up for good mental and physical health.

“In most cases people care because they want to and because they are deeply committed to someone they love. Caring in and of itself is not the problem here. But we need to do a lot more to support people in this position so they can continue to stay fit and well, and so they have the time and space to enjoy living their own lives, while helping someone else to live theirs.”