Almost a third of long-term carers (32%) have not had a day off in five years, according to a new report.
The study found a lack of respite available for the 3,841 people surveyed who have been caring for five years or more.
Among all those who have been caring for a year or more (5,168), 40% have not had a day off from caring for more than 12 months.
The report, from the charity Carers UK, warned that people were at "breaking point" due to the lack of support they receive, while some have seen a cut in the social care offered to them.
The study comes as new figures from the Office for National Statistics (ONS) show that 8% of the UK private household population are informal carers for another person.
Of these, 59% are female and 29% spend 35 hours a week or more on caring.
This is often on top of holding down a job. Half of adult carers are employed either part-time or full-time.
When women reach 50, they are likely to spend 5.9 years of their remaining life as unpaid carers, the figures also showed.
At 65, they spend 2.6 years as unpaid carers.
In contrast, men aged 50 are likely to spend 4.9 years of their remaining life as an unpaid carer and, at age 65, it is 2.7 years.
In the Carers UK report, 68% of current carers who have been caring for over a year and have not had a day off for more than a year say their physical health has worsened.
Meanwhile, 72% said their mental health has suffered.
Overall, 78% of more than 6,000 carers surveyed felt stressed about their role, and 72% said caring had made them feel more anxious.
Almost seven in 10 (69%) struggled to get enough sleep due to caring, while 45% have found it difficult to keep up a balanced diet.
Nearly half of respondents (46%) said they have suffered from depression because of their caring role.
The charity's report, State of Caring 2017, also found that carers most frequently listed access to breaks as the factor which could make a difference in their lives.
Yet only 16% of carers were currently buying or receiving a break from caring in the form of respite services or alternative care.
When asked about barriers that stop them taking a break, 31% said they worried about the cost of care while they had a break, 31% said the person they care for is unwilling to accept care from others and 27% said there was a lack of specialist support on offer.
A fifth (19%) had low confidence in the quality of care available.
When it came to social care services, 29% were worried that practical support for them might be reduced in the future.
Already, more than half (59%) of carers said there had been a change in the services they received and, of these, 39% had seen a reduction in support.
Helena Herklots, chief executive of Carers UK, said: "More and more of us are stepping in to provide care and support to loved ones and doing so for more hours every week.
"Without access to breaks, carers can quickly reach breaking point, unable to look after their own health, nurture relationships with friends and family or have the time they need to themselves.
"Our research shows that carers are struggling to get a break because appropriate support for their loved ones isn't available or services they rely on are being cut or charged for.
"The need for an action plan from the Government on how they will improve support to carers is now urgent.
"Increasing funding for carers' breaks is a key part of the change needed to support people to care without putting their own lives on hold.
"Given the enormous value of unpaid care provided by the UK's 6.5 million carers, estimated to be worth £132 billion each year – getting some time away from caring to spend time with a partner, get to a medical appointment or just get a full night's sleep surely isn't too much to ask."
A Department of Health spokesman said: "Carers do an incredible job supporting others and we want to make sure they feel supported too.
"That's why we gave carers more rights through the Care Act and the Better Care Fund includes £130 million to fund carers' breaks.
"We are committed to improving social care which is why we will consult later this year and put it on a stable footing for the future."