That’s challenging for anyone, though the extent of what’s called “caregiver burden” remains a subject of debate. For years, researchers have presented caregiving stress as a potential source of depression, compromised health, even premature death.

Some of those findings are being reassessed; a recent study in The Gerontologist, for instance, argues that the picture is “overly dire” and that several studies find benefits for caregivers — the “healthy caregiver” hypothesis.

(My own hypothesis: Both responses can be true, with caregiving providing purpose and activity in some situations but proving exhausting in others.)

People over 75, however, can find caregiving particularly taxing. They spend an average of 34 hours a week on caregiving tasks, the National Alliance for Caregiving report found, 10 hours more than caregivers over all, and they are less apt to have other unpaid help.

Because 46 percent take care of spouses, they are also more likely to be live-in caregivers, a known source of strain. “You’re responsible for their safety,” Ms. Hunt said. “You have to interact with the person all day long and maybe all night long, so your sleep is disturbed. It’s more stress than if you’re coming over periodically to help.”

The typical older caregiver in the study had been providing care for over five years.

“Just the physical part of it, the lifting and bathing and all of that, can hurt you,” said Donna Wagner, dean of the College of Health and Social Services at New Mexico State University and a longtime researcher on family caregiving. Older caregivers, typically women, have their own health issues.

“My mother is a perfect example,” Dr. Wagner said, describing her as a 4-foot-10 woman who cared for her husband, an obese man with heart disease and diabetes, for seven or eight years. “I don’t even know how she managed,” Dr. Wagner said. After he died at 85, “it didn’t take her long to slide right into dementia. It’s as if she put it off while she had responsibility for him. There are a lot of mysteries.”