Continuous 14-day heart monitoring of 27,658 UK Biobank participants (average age 71) detected previously undiagnosed atrial fibrillation in 3.2% of men and 1.7% of women, with 68% showing intermittent episodes. Notably, 27.4% of cases emerged only during the second week, highlighting how brief monitoring periods miss critical diagnoses. Ventricular tachycardia appeared in 12.1% of participants, though sustained episodes remained rare at 0.4%. The study revealed distinct circadian patterns: atrial fibrillation peaked during nighttime inactivity while ventricular irregularities increased with daytime physical activity. This represents the largest population-based assessment of extended cardiac rhythm monitoring, providing unprecedented insights into how common subclinical arrhythmias actually are in older adults. The finding that over a quarter of atrial fibrillation cases required two weeks to detect challenges current shorter monitoring protocols used clinically. However, as this preprint awaits peer review, these prevalence estimates and clinical implications require validation. The integration with UK Biobank's extensive health data promises to illuminate how these hidden rhythm disturbances affect long-term cardiovascular and neurological outcomes.