The problem with most correlational studies is that there are too many confounding factors to be able to draw meaningful conclusions from the results. Yes, people who shift their sleep schedules later on weekends have higher rates of heart disease. Yes, you can control for things like income, employment, and education, all of which correlate with social jet lag—if you have a job that keeps regular Monday through Friday hours, you're more likely to indulge in a Saturday morning lie in. But what if people who sleep late on Saturdays also drink more? That seems pretty likely. And we know that heavy drinking can cause heart problems. Or what if the layabouts also tend to smoke more? Or tend to be more overweight and eat junk food while they're pulling late nights? Those things all increase your risk of heart disease—independent of your sleeping habits.