When Apple Watch detects that your heart rate has increased while you have been at rest for the last ten minutes, you will be notified immediately. Apple has set the alarm level at 120 beats per minute. If you want to increase or decrease the heart rate at which you will be notified if you are at rest, do the following: open the Watch app on your iPhone and select the My Watch tab in the lower left. Select Heart Rate > Elevated Heart Rate and the beats per minute that will be the alarm limit.