Abstract Background We have devised a novel psychological intervention, cognitive behavioral therapy for paroxysmal atrial fibrillation (AF-CBT), that targets symptom preoccupation and avoidance behaviour. AF-CBT significantly improved cardiac anxiety and quality of life (QoL) in a randomized trial. To what extent this may be attributed to changes in physiological parameters is yet to be determined. Purpose A) to assess changes in objective and subjective measures of physical activity and sleep. B) to evaluate potential treatment effects on heart rate variability (HRV). Methods Patients with paroxysmal AF on optimal medical therapy (65.4±8.3 years, 58% women) and European Heart Rhythm Association Symptom Score ≥2b were randomized to a 10-week therapist-led online AF-CBT (n=65) or a control group (n=62), with the primary outcome of Atrial Fibrillation Effect on Quality-of-life (AFEQT). A Patch-Holter with an integrated accelerometer was applied for five days at baseline, post-treatment, and at three months follow-up. Each recording was divided into day and night (sleep time) based on posture, heart rate, and activity. HRV was analysed in both time and frequency domains, excluding AF episodes and aberrant beats. Results AFEQT increased from 62.4±14.3 to 83.7± 13.8 points in the AF-CBT group at the 3-month follow-up with a relative difference of 15.0 points. At baseline, the participants walked 8040±2600 steps/day and slept 8.0 ± 1.1 hours. AF-CBT did not increase physical activity, and there were no significant changes in objective sleep duration. The Insomnia Severity Score went from subclinical (12.2±6.7) to near normal values (8.1±5.6), which was significant as compared to controls (p=0.032). AF burden increased from 5.3% to 7.3% in the AF-CBT group and 5.0% to 9.2% in the control group, with no difference between the groups (p=0.59). We did not observe any change in heart rate or HRV indices at rest during the day or at night. Conclusion AF-CBT did not mediate its effects on QoL by improving objective physiological variables. Thus, our findings underline the importance of subjective, psychological, and behavioural aspects and indicate that AF-CBT might fill a void beyond medical and lifestyle interventions in comprehensive AF care.