I did a brief search because I was curious who was remembering correctly here. There have been several studies correlating "long" sleeping with mortality, but the study I found that attempted to control for confounding variables[1] seems to suggest that most of the differences in mortality in the populations studied can be explained by "depression and low socioeconomic status". It seems like a lot more study has been devoted to undersleeping than oversleeping, and I can't find anything convincing as far as maximum healthy sleep time and effects of over-sleeping. One study I still want to look at but haven't gotten around to yet is the stuff the NSF produced in 2015 when making its recommendations about sleep duration.
It appears there is indeed a link between short/long sleep and mortality. As you said, the possible causes are usually not reviewed. This concerns me as I do sleep 9h+ on average. (But then again I do have an eye-illness that require a lot of concentration to overcome)
> Conclusion: Both short and long duration of sleep are significant predictors of death in prospective population studies. > (...) > Future studies should be designed to answer the question whether sleep duration is a cause or simply a marker of ill-health. https://academic.oup.com/sleep/article/33/5/585/2454478?sear...
> Long sleep was significantly associated with mortality, incident diabetes mellitus, cardiovascular disease, stroke, coronary heart disease, and obesity. https://linkinghub.elsevier.com/retrieve/pii/S1087-0792(17)3...
I read a few studies when i started bodybuilding and that seemed to be the consensus
Also undersleep affects cognition to a signifiant degree, iirc it was 40% diminution day one, 60% day 2 on memory tasks (and you have the same health issues as oversleep, perhaps to a higher degree)