Millions of Americans will have a hard time falling or staying asleep tonight, and research says most of them will be women. “Insomnia is definitely more common in females, and it seems to begin fairly early on,” said Dr. Meir Kryger, a professor at the Yale School of Medicine who studies sleep. Sleep problems can appear in women as early as their teens or 20s, he said. Various research shows women are more likely than men to experience the sleep disorder. Women are about 1.5 times more likely to have insomnia, said Kryger, who has written several books on the topic, including “The Mystery of Sleep,” which was published in March.