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...