Atrial fibrillation (AF) is usually understood as the end result of growing electrical disorder in the heart. The transition into AF is therefore commonly treated as a stochastic process: rhythm becomes increasingly irregular until fibrillation begins. In that framework, the small fluctuations preceding onset are usually treated as noise. The present study tests a different possibility: that part of this apparent noise is structured. Unified Substrate Theory (UST) predicts that systems approaching critical transitions can carry discrete oscillatory corrections in logarithmic time, with a fundamental angular log-frequency β = 4 and higher harmonic modes at β = 9, 16, 27.14, and beyond. The present work tests the fundamental mode.