You do not seem to be talking of the same thing. He wrote:

> It may be indicative of a health issue

Now you write:

> No study I've seen seems to suggest there's any problem with sleeping more up to something like 12 hrs/night I think

So he said it may be an indication of an issue, and you write that it may not be unhealthy to sleep a lot. Those are two different things.

As for your claim: A quick search returns many hits related to issues caused by sleeping more than ~9 hours. I have not reviewed them (I spent 1 minute on this) so can't say if the studies well performed. Example:

https://www.ncbi.nlm.nih.gov/pubmed/19473367 "Prolonged sleep duration (night-time sleep and daytime napping) may be associated with an increased risk of dementia."

But saying that no study would even suggest there would be a problem seems strange considering I found a bunch of them in a minute. Maybe you have done a deeper review of the studies and concluded that they are invalid, or?

I think the key here is "associated". Sleep deprivation has also been associated to dementia.

Yes, the link I gave pointed that out. I just questioned that he hadn't found a link since there are so many studies available. If there are associations between sleeping lock and many illnesses then of course sleeping long could be an indication of an issue.

IIRC there are negative effects of oversleeping; some mininal health issues, but cognition is the same as a 8-9hour sleep

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)

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.

[1] https://academic.oup.com/sleep/article/29/7/881/2708387

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...