> There was little evidence to support associations of increased sleep duration with ... Alzheimer's disease risk [OR = 0.89 (95% CI = 0.67-1.18); P = 0.41].
Pg. 858:
"Similarly, short and long sleep duration and poor sleep quality have also been linked with an increased risk of dementia. Although a similar J-shaped association was observed in our observational analysis, we were limited to performing only the linear MR analysis, as the non-linear MR method requires a large number of cases and individual-level data. In our linear MR analysis, we found no clear evidence that an increased sleep duration was associated with a higher risk of all-cause dementia in UK Biobank or with AD in IGAP. This is unsurprising, as the true association might be non-linear and we were limited with only 1343 dementia cases in UK Biobank. Also, IGAP does not capture non-AD dementia types and comprises an older and more heterogeneous population."
"Results for risks of dementia and AD are still too imprecise to draw any definitive conclusions. Our findings suggest that, in clinical care, attention should be paid to sleep-duration patterns and improved sleep habits could represent a potential therapeutic target for cognition."
I assume a causal analysis would also determine the direction of the causality, e.g. if it turns out that people predisposed to dementia have more trouble sleeping in middle age. That definitely seems like a tall order and probably not exactly what the linked study can answer.
The best causal analysis is interventional, but many times (such as in this case) it would be both extremely challenging and likely hard to do in any ethical fashion (I can't imagine how it could be done, but perhaps someone could).
Short of that, you're left with instrumental variables. Genetics offers a very nice instrumental variable, since the causal arrow largely goes one way if you setup your study design carefully.
That is generally performed as Mendelian randomization, which has several assumptions but, if met, has nice properties. https://en.wikipedia.org/wiki/Mendelian_randomization