Atrial fibrillation is the most common cardiac arrhythmia and is characterized by an irregular and fast heartbeat. Patients with atrial fibrillation can experience symptoms as palpitations, fatigue and shortness of breath and are at increased risk for stroke and heart failure. Many symptomatic patients need to undergo electrical cardioversion to relieve their symptoms. During this procedure, an electric shock is applied to the patient’s chest to restore the heart’s normal rhythm. Electrical cardioversion is performed in the hospital under general anaesthesia because of pain and needs to be repeated in many patients due to the recurrent nature of atrial fibrillation. Researchers of the Leiden University Medical Center in the Netherlands have developed a method that allows the heart itself to generate the electrical current needed to terminate atrial fibrillation. This has been made possible by introducing light-sensitive proteins in the rat heart through optogenetic gene therapy. So-called vectors are locally applied on the atria of rats through a small thoracic incision, after which light-sensitive ion channels are formed in heart muscle cells. Exposure of these ion channels to blue light results in the generation of bio-electrical current. In collaboration with the Delft University of Technology in the Netherlands, the researchers next implanted a small LED device near the heart. This LED device is connected to a cardiac rhythm monitor. Upon detection of atrial fibrillation, the cardiac rhythm monitor automatically activates the implanted LED device. The bio-electrical current that is produced in the atrium by activation of the light-sensitive ion channels restores normal cardiac rhythm. Although further research is needed before this approach can be used in humans, this study can pave the way for the development of pain-free cardioversion therapy for patients with atrial fibrillation. By restoring cardiac rhythm in a rapid, continuous and completely automated fashion, this therapy has the potential to improve patients’ prognosis and quality of life.