Stewart tells the magazine the stress of working in the industry affects her weight, eating habits, and sleep patterns.
"I’m a little bigger than sample size when I’m eating cheeseburgers and am happy and comfortable," she admits. "If I’m stressed or working, the weight falls off. My weight and my sleep are tied to my nervous system. Sometimes I’ll sleep for 12 hours a night and sometimes sleep just doesn’t exist for me for a couple of months."
The actress also opens up about filming sex scenes for movies, adding that she only hates them "when they’re contrived."
"On Twilight we had to do the most epic sex scene of all time," she recalls, referencing Bella and Edward’s long-anticipated honeymoon in Twilight: Breaking Dawn Part 1. "It had to be transcendent and otherworldly, inhuman, better sex than you can possibly ever imagine, and we were like, 'How do we live up to that?' It was agony. Which sucks, because I wanted it to be so good."
The June 2015 issue of Harper’s Bazaar U.K. is on newsstands April 30.