ELI5 HRV - what is heart rate variability and why can a steady heartbeat be trouble?

Why does a perfectly steady heartbeat actually serve as a warning sign you should not ignore? If your heart is a drummer, why is a little bit of "bad timing" a good thing? How can your smartwatch tell you are stressed before you even feel it? Why does splashing cold water on your face trigger an ancient, "hidden" survival reflex? Why did Neil Armstrong’s heart race to 150 BPM while his co-pilot stayed totally calm?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: iacuriousguy1968, quesoqueso, elderberry2, spooky_fellow, swolemedic, uptowntwerk, narwhalondrugs and alternative-sea-6238

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