The Great Gatsby star's elderly relative Margaret, affectionately known as Nans, was diagnosed with Alzheimer's disease in 2004 and Carey has opened up in the past about feeling helpless and she watched her grandma's mental health deteriorate over the years.

Nans currently lives in a care home in Wales, and although visits have been tough at times, the actress insists she can tell her 91-year-old grandmother appreciates the company.

"Every visit for the last seven years, she hasn't recognised any of us," Carey explained during a recent guest-hosting stint on BBC Radio 4's Best of Today show. "When we leave, she won't remember that we've been there. But... there's a calmness and there's a companionship and these really fundamental feelings of being loved and being taken care of by people who, you know, family who really love you."

The 31-year-old, who is married to folk star Marcus Mumford, reveals she and her loved ones have started using music to try and trigger Nans' memory and the form of therapy has shown promise.

"We've had terrible visits where we've all ended up in tears but then we have the visits where something really magical happens," she said. "She was a great lover of music and she taught me to sing and she taught me to play the piano and we realised that a lot of the times, just playing music and sitting with her was just the sort of loveliest time that we could spend with her.

"Music is something that has often come around for people who have dementia that it's a way of linking to the past, it's a nostalgic thing, it's a calming thing."

Carey was appointed the Alzheimer's Society's first U.K. Global Dementia Friends Ambassador earlier this year (16), and at the time, she revealed she wanted to use her platform to draw attention to the importance of making communities dementia-friendly.

"It gets tiresome hearing dementia being the butt of a joke," she sighed. "I think there's a general misunderstanding in a lot of areas that dementia is a natural part of ageing or it's just something that happens to you when you get older."

She continued, "I used to grow up hearing a lot of people referring to their grandparents having 'lost their marbles,' which is of course something that we'd never say about somebody who'd had cancer or heart disease. I think the understanding that dementia is a disease - it's a disease of the brain, there are lots of different kinds of dementia, Alzheimer's is one of them - and just spreading that awareness so that people really understand that this is a disease we have to fight."