Atrial fibrillation (AF) is the most common heart rhythm abnormality in the general population. Current recommended methods to maintain sinus rhythm or reduce AF burden (% time a person is in AF) in patients with non-permanent AF are costly and ineffective. Hence, there is an urgent need to discover novel inexpensive strategies to reduce AF burden. It is well-established that regular aerobic exercise reduces cardiovascular morbidity and mortality; however, adherence to regular exercise is poor rendering it an ineffective public health strategy. Evidence is emerging to suggest that traditional moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT) may be associated with lower AF burden. Compared with traditional MICT, HIIT is possibly more time-efficient; hence, it may promote adherence. Since lack of time is the most common reason for poor adherence to regular exercise, the time-efficiency of HIIT holds promise to be a "game-changer". However, many knowledge gaps remain. First, HIIT has never been compared directly with MICT in relation to AF burden; thus, whether HIIT is comparable or superior to MICT is unknown. Second, the time commitment required for a "conventional" HIIT program (120 mins/week) is not substantially less than the recommended MICT (150 mins/week); hence, it is doubtful that it can be a real "game-changer" in terms of promoting adherence. Third, HIIT performed using a cycle ergometer is a safer alternative to the treadmill in older adults, but has not been evaluated in patients with AF. Finally, mechanisms underlying the salutary benefits of exercise in relation to AF burden remain unknown. To address these knowledge gaps, the investigators will evaluate an innovative HIIT program—sprint interval training (SIT)—using a cycle ergometer that only requires 10 minutes per session and 30 minutes per week, in contrast to the "conventional" HIIT program that requires 40 minutes per session and 120 minutes per week. In this pilot randomized controlled trial (RCT), the investigators will randomize 60 subjects with non-permanent AF (1:1:1) to SIT vs. MICT vs. no exercise control. At enrollment, these subjects will undergo heart rhythm monitoring by a non-invasive ambulatory heart rhythm monitor, V02 max testing, assessment of cardiac size and function by cardiac MRI, and assessment of AF symptom severity; these measures will be repeated at 3 months.