This looks interesting: The 90-Minute Baby Sleep Program: Follow Your Child's Natural Sleep Rhythms for Better Nights and Naps, by Polly Moore.
Book Review from ParentDish.
Dr. Moore was a sleep researcher who got pregnant and thought, "No problem, I'm an expert at sleep." Then her babies showed her who's boss. Her nursery became her sleep lab and she noticed something that wasn't talked about. 90-minute sleep/alertness cycles.