I simply do not know how one concludes that sleep, among all other things, is the reason for the deaths? When studying humans, how do you account for all the ot

I simply do not know how one concludes that sleep, among all other things, is the reason for the deaths? When studying humans, how do you account for all the other lifestyle variables? Does anyone on HN do these types of studies and can comment? Seems so very unscientific.