[Tenoke]

For example all-cause mortality in the linked there biggest meta-analysis is actually lower at 6 than 8 hours (graph[0]) and only slightly higher than at the optimal point. This isn't the same as general ability but it's likely correlated and all the evidence seems to point in the same direction.

0. https://guzey.com/files/books/why-we-sleep/shen2016.jpg

[joconde]

How do we know that shorter sleep causes lower mortality, instead of, say, that better health or some genes cause both?

Even if we assume that correlation is causation, I haven’t seen much of that evidence.