My Apple Watch flagged possible atrial fibrillation. What should I do right now?

Apply the symptom test immediately: are you experiencing chest pressure or chest pain, lightheadedness or near-fainting, or shortness of breath at rest? If yes to any of these, call emergency services now. Do not drive yourself. Do not search Reddit. These symptoms alongside a cardiac alert indicate a potentially hemodynamically significant arrhythmia that requires immediate evaluation.

If you have none of these symptoms: document the time, what you were doing when the alert occurred, save any ECG trace generated through the digital crown function in your Health app, and call your cardiologist's office when it opens. A single asymptomatic notification in a healthy 48-year-old with no cardiac history is a conversation to have during business hours, not a 3 AM emergency room visit. Two notifications within a month is a pattern that warrants a Holter monitor. The Apple Heart Study found that among patients who received an irregular pulse notification, 84% were confirmed to have AFib on patch monitoring, so the alert is credible and worth pursuing, just not at a pace determined by panic rather than clinical urgency. (Perez et al., JACC, 2019)

Cardiologist's calibrated position, Solid (1) for this triage framework. The symptom test is the universal first step.

What to do: Save this answer somewhere accessible. The 2 AM moment when you receive an unexpected cardiac notification is not the moment to remember the nuance. Having the symptom test memorized is more useful than knowing the specifics of the algorithm.

For the full picture, read What Your Apple Watch Is Trying to Tell You.