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)
> 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...