The 26-year-old, who married British comedian Russell Brand last month, said, 'If I don't get eight hours of sleep a night, I'm a bitch.
'I know that I'm strong. Nobody says no to me, ever. There's no-one in control of me. I have my head screwed on. I'm not an idiot.'
Addressing rumours that she's been offered £210,000 a week to judge the US version of The X-Factor, the singer told Cosmopolitan, 'I think Simon Cowell is flirting with me through the press. He is a big flirt. I know a lot of it is rumours.
'But sometimes rumours turn into reality.'