My Garmin (Vivosmart 4) band tracks "stress" and "body battery" metrics, which are based on your Heart Rate Variability. For the first couple of days after my first mRNA shot there were no anomalies at all, but then out of nowhere my stress levels went through the roof (and my body battery completely flatlined) in a way that I've never seen before. This lasted for 6 full days, now everything seems to have returned to baseline..

I didn't feel anything about the whole ordeal though, apart from some sore muscles in the first days. Crazy interesting to quantify the amount of stuff going on in the background without you being aware of it.

EDIT: Specifically, during these 5 nights my stress level was reportedly _unmeasurable_ for about 50% of the time. Unmeasurable is common when you are physically active (making the PPG data too noisy to properly calculate normalized RR intervals) but very rare during sleep, so this probably indicated a very high amount of irregular/ectopic heartbeats.