Although this isn't directly relevant to your question, I was listening to an article on the radio talking about top tier cyclists needing to have a HR monitor with an alarm while they sleep. Apparently their resting HR is so low that they can die in their sleep. When their HR drops too low the alarm goes off and they have to start exercising to increase their HR.
I'm relatively new to biking and ride a torque sensing mid drive ebike that allows me to get just the exercise I want. Although I am very athletic for my age (63), I'm not a fan of stressing my heart too much. I have no real scientific reason behind it. Just a gut feeling that it isn't as good for your general health as many might think.