The 27-year-old regularly tops the beauty polls but admitted she has little self-confidence in her looks.
Asked if she thinks she's pretty, she said to UK's Cosmo magazine: 'No. I don't think anybody thinks that of themselves. It's odd. I often wonder how I look to other people.
'I still have my ugly days - every woman does. I refuse to believe anyone who says they don't.'
The X Factor judge said she does have a beauty regime but admitted that she sometimes forgets to take off her make-up before bed.
She said: 'I try to [take it off]. I like to let my skin breathe. But the other night I fell asleep on the couch with a full face of make-up and woke up with mascara all over my pillow, which is never good.'