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.