Madonna has admitted that she is often "sad" or "upset".

The singer stars in the December issue of Interview magazine and she is quizzed by friend and magician David Blaine.

Speaking about one strand of songwriting inspiration she has, Madonna said:

"My children. Stuff they say. Like, my son came to the studio the other day and said he didn't want to go home. I said, 'No, you have to go home; it's time for bed'. And he said, 'Mom, I feel like I'm isolated from your heart'. I was like, 'Oh, that's a good line. I have to use that'.

"You get it everywhere, the high and the low. You've got to be open to it all. Suffering is a big informer, a big catalyst for creation. You take your sadness, your despair, your sense of injustice, and you put it in your work."

When asked by Blaine if it is easier to write when sad or happy, the Queen of Pop replied:

"Sad [laughs]. Unfortunately. But it's okay, because I'm sad most of the time, so we're good. Not sad, but upset about something."

Madonna meanwhile posed topless for the magazine's accompanying photoshoot.

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