A lot of it boils down to blood pressure. High blood pressure is a serious contributing factor to cardiovascular incidents (as well as a slew of other negative health risks), and getting a good night's sleep will help keep blood pressure down. This is also why the amount of heart attacks are up around 24% after daylight savings[1]; an hour less sleep means higher blood pressure means higher risk of heart attack (relative to any other 'normal' day).
I can definitely see how the same logic could apply to Mondays. Less sleep, more stress = higher blood pressure = higher risk of heart attacks.
[0] https://www.goodreads.com/book/show/34466963-why-we-sleep
True but checking your blood pressure is painless, basically free and so easy that one can do it at home with no loss of precision.
Speaking as someone with mild high blood pressure, I see people obsess over diets, physical activity, looks, that never go to the doctor or check for their health conditions and "cure" every discomfort/pain with painkillers or ibuprofen.
And they of course all have some advice to give to me to improve my condition based, of course, on some diet they read online or to try yoga or acupuncture (or whatever is fashionable at the moment) and totally ignore the fact that I've been checking my blood pressure for over 20 years, I know a thing or two about it, because doctors. Yeah... I am that crazy! I see doctors!
Once a year is more than enough for people that have never been diagnosed with anything and yet very few people regularly do it, even here in my Country where medical checkups are virtually free.
IMO the real silent killer is indolence.