Wearable ECG devices reveal that AI-calculated heart age follows distinct circadian rhythms, with healthy individuals typically showing nocturnal recovery patterns where their electrocardiographic age drops by more than 0.6 years overnight. Analysis of 122 participants found that those with disrupted circadian patterns—lacking this restorative nighttime decline—had 6.09 mL/m² higher left atrial volume and four times greater odds of severe atrial enlargement. This finding represents a novel application of continuous cardiac monitoring beyond traditional rhythm detection. The discovery that circadian electrophysiological patterns predict structural heart changes independent of conventional risk factors opens new avenues for preventive cardiology. Disrupted circadian recovery may signal early atrial myopathy before clinical symptoms emerge, potentially identifying individuals at elevated risk for atrial fibrillation or heart failure. However, this preprint awaits peer review, and the moderate reproducibility of phenotype classification suggests refinement may be needed. The research is paradigm-shifting in demonstrating that timing of cardiac aging matters as much as absolute values, though larger studies must validate whether these patterns truly predict clinical outcomes rather than merely correlate with subclinical changes.