Catherine Zeta-Jones, who has been by husband Michael Douglas's side during his cancer fight, has sought mental health treatment for herself, her rep says. "After dealing with the stress of the past year, Catherine made the decision to check in to a mental health facility for a brief stay to treat her Bipolar II disorder," the rep says in a statement. "She's feeling great and looking forward to starting work this week on her two upcoming films." Zeta-Jones, 41, spent about five days in the unidentified facility, a friend tells PEOPLE. Bipolar disorder, a mental illness marked by elevated or irritable moods alternating with periods of depression, afflicts about 6 million Americans. Those with Bipolar II tend to have more depression, with the mood swings spread over a longer time and the "up" periods less elevated. The episodes can be triggered by major stress or life changes. "There's no question it's been a stressful year," the friend says. "Catherine has had deal with Michael's illness and that's been hard. She went in for a few days because she's about to start working, and wanted to make sure she's in top form, which she is." Douglas, 66, announced in January he was declared free of cancer after what he called a "wild six-month ride" of chemotherapy and radiation for a tumor on his tongue. "The hardest part is seeing his fatigue, because Michael is never tired," Zeta-Jones told PEOPLE last year when Douglas was in the midst of his cancer battle. She said Douglas told their children Dylan, 10, and Carys, 7, the difficult news himself: "Daddy sat them down and told them he has cancer now."