(Edited, see replies) Also "However, various health and lifestyle factors could be playing a role in the links they identified." I imagine that if someone is

(Edited, see replies) Also "However, various health and lifestyle factors could be playing a role in the links they identified." I imagine that if someone is poor (on average) they'd get less sleep due to living in noisier areas, working longer hours and so on. I also imagine being poor would decrease life expectancy on average. And another one I thought of ... 1990-2017 is not a long time. Older people (say >50) tend to sleep fewer hours. Older people are much more likely to die in a 30 year period than younger people (<30), so there is that. I wonder if that was taken into account.