Couldn’t get to sleep last night? If so, you’re not alone. Millions of Americans complain of insomnia at least a few nights a week. What causes it and what are the health risks associated with the condition? We’ll spend this hour with Gayle Greene, a member of the American Academy of Sleep Medicine and the patient representative on the board of the American Insomnia Association. Greene’s new book is “Insomniac” (University of California Press, 2008).