Multifractal Analysis Reveals Decreased Non-linearity and Stronger Anticorrelations in Heart Period Fluctuations of Fibromyalgia Patients.By Cesar F. Reyes-Manzano, Claudia Lerma, Juan C. Echeverría, Manuel Martínez-Lavin, Laura A. Martínez-Martínez, Oscar Infante and Lev Guzmán-Vargas