How to comfort you're wife who has nightmares ?

Me and my wife have known each other since we were kids she's my best friend when she was little she used to have really strange dreams now they're coming back to her. She said some are good some are scary but she said they seem so real she can almost hear everyone and everything in them. She use to get them maybe once or twice a year now it's all the time. Nothing bad has ever happened to her it's been a great life for her. She doesn't understand why she's having these dreams especially now. She loves scary movie's but she doesn't watch them all the time I don't think the movies would have nothing to do with it. If there really bad she wakes up at night crying and doesn't want to go back to sleep. Shes not pregnant or on her period. How can I comfort her ?