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
> In the Northern Hemisphere, the switch to daylight savings time in March results in most people losing an hour of sleep opportunity. Should you tabulate millions of daily hospital records, as researchers have done, you discover that this seemingly trivial sleep reduction comes with a frightening spike in heart attacks the following day. Impressively, it works both ways. In the autumn within the Northern Hemisphere, when the clocks move forward and we gain an hour of sleep opportunity time, rates of heart attacks plummet the day after.
I don't see a specific study cited, but my ebook copy doesn't seem to have all the footnotes.
I pulled up the article you linked on sci-hub: https://sci-hub.se/https://pubmed.ncbi.nlm.nih.gov/18971502/ The only use of the number "24" refers to hours in the day, and its summary doesn't seem to match your claim:
> The incidence of acute myocardial infarction was significantly increased for the first 3 week-days after the transition to daylight saving time in the spring (Fig. 1A). The incidence ratio for the first week after the spring shift, calculated as the incidence for all 7 days divided by the mean of the weekly incidences 2 weeks before and 2 weeks after, was 1.051 (95% confidence interval [CI], 1.032 to 1.071). In contrast, after the transition out of daylight saving time in the autumn, only the first weekday was affected significantly (Fig. 1B); the incidence ratio for the whole week was 0.985 (95% CI, 0.969 to 1.002