> The difficulty is that on average the people who sleep well and are physically active are also the people who eat well, have higher incomes and better access

> The difficulty is that on average the people who sleep well and are physically active are also the people who eat well, have higher incomes and better access to healthcare Truly effective causal studies are still in their infancy, relatively speaking, and are still prone to the same correlation//causation fallacies. For instance, it's no fun studying things that may be the root cause of those behaviors but is hard to quantify, like "go-getter-ness" or something similarly illegible to scientific methods. You don't really know the structure of the problem so you can't put useful priors on it and focus in toward a reasonable inference. It's much easier to run t-tests and ANOVA by writing a couple lines of R so that's where we've settled as a community.