The commenter may have been referring to Rendell's Death of Marco Pantani which IIRC had a line that went something like "during the day he lived to ride, at night he rode to live."

It was detailed in a biography of Marco Pantani - referring to him and his peers. This was probably partly (at least) related to epo - the increased oxygen carrying capacity of the blood was probably what led to their exceptionally low heart rates.

I’ve not read about this phenomenon outside of the epo-dominated era.

Paul Kimmage wrote about it in his book:

There was the confession of the top Italian‘Y’and his explanation ofhow he had narrowly cheated death after a stage of the Tour of Italy.Boosted before the stage by an injection of EPO, he had gone to bed hat night and slept peacefully for two hours, unaware that the oxygen-enhanced blood, flowing through his veins, was rapidly thickening to treacle. EPO is transformed into a lethal cocktail, not during a race when the blood is pumped around the body by a 180 beats-per-minute, high-revving, super-fit, heart rate but at night when the revsdrop way below the norm. As Y’s pulse dropped to a low of twenty-fivebeats per minute, his blood began to clot and his heart began to stall.Had he not been sharing with a team-mate, there is every chance they would have found him dead in the morning...Y lived to tell the tale.Others were not so fortunate.

and Robin Parisotto wrote this:

Between 1987 and 1990, 18 cyclists died tragically and suddenly all from heart attack or stroke. EPO was known to thicken the blood the common cause of heart attack or stroke. Many victims developed clots that broke off and travelled to their hearts or brains; others died of simple cardiac arrest, the organ struggling to pump blood the consistency of oil.

These are all from [1], which is a review of the evidence as to if this actually occurred. It cast doubt on it, but I've read enough accounts to think that cyclists did believe it occurred and probably were wearing HR monitors in bed when on EPO.

[1] https://www.tandfonline.com/doi/pdf/10.1080/17460263.2011.55...

Not sure about dying but here is my personal experience. I am far from being anywhere close to a real cyclist. Still I used to ride a bit too much and at some point my resting heart rate dropped down to below 45. At this point I got really scared and cut down on my cycling. Now it is more like 50 so it is ok. Funny thing I also swim a lot but it does not seem to affect my heart rate as much as cycling has.

There's a chance this tale is apocryphal, but the 90s was a pretty dark time for professional cycling and it is of course 100% drug use (EPO etc) related. I've heard this story in several cycling books/pro cycling talks usually in connection to Marco Pantani. Marco Pantani stories are often exaggerated as the man was almost like a folk hero to many fans and sad stories about his death historically sold well especially in Italy. There are a great many "legendary" stories regarding EPO use in the pro peloton in the 90s and riders were often deeply paranoid bordering on superstitious about what the training and new drugs were doing to their bodies, hence the rich vein of apocryphal tales and dubious advice.

To be clear, it's definitely not a practice that is at all required of 99% of healthy pro-cyclists! Top cyclists wear heart rate monitors to train more efficiently just like many other athletes. I could certainly believe some riders would have been _paranoid_ enough to wear one regardless of merit in bed while taking EPO though. These were often young men heavily pressured into injecting for first time etc and in 90s not like you could just google "EPO side effects" like you might today.

EPO use was rampant in cycling the 90s, largely because there was no good test to detect it widely used in sport till early 2000s, and the advantage it could confer was pretty significant in cycling - see Lance Armstrong's career.

Could be a bit of both?

I’ve heard of EPO-users getting up in the middle of the night to trainer cycle because exercise increases blood flow which reduces clotting risk. Lower heart rates may cause/compound this.

Even a non-fatal minor clot in a leg or lung will doom a cyclist.

In hospitals, sedentary patients will receive anti-clotting drugs, but may not (or get reduced dosages) if they’re able to walk around a lot.

Wouldn't the pumping of thicker blood cause heart rate to become lower? This seems like a lurking variable situation.