The night Yael Levy turned a circle of menacing sharks into a ring of harmless dolphins, she knew she had achieved mastery over a life-long foe: her nightmares. "I was able to change my nightmare while it was happening," says the 29-year-old New York City graduate student. "I had control over my dreams." That relief was more than Levy expected when she showed up at the Sleep-Wake Disorders Center at Montefiore Medical Center two years ago. Levy says she just wanted help for her insomnia. She had no idea her nightmares were treatable, too.