> I don't know how true it is, but I've heard that some cyclists had/have to set alarms throughout the night so they wake up to do jumping jacks to get their heart rates back up above the "artificial" 15bpm that their training + cocktail of drugs has caused.

Even as a casual runner (top 20% in a marathon, def not competitive), I have had to silence low heart rate alarms on my apple watch because my heart rate regularly drops below 40bpm at night. When I’m in peak condition for a race, I’ve seen it drop as low as 30.

Professional cyclists have the highest VO2max of any athlete. Even without drugs I would be completely unsurprised if their heart rate gets to 20bpm in deep sleep.