Sleep duration vs sleep quality — which matters more for longevity?

This is a case where the more intuitively appealing idea has the weaker evidence. Sleep duration's mortality link is one of the most thoroughly meta-analyzed findings in sleep research: multiple independent, large dose-response meta-analyses consistently find a U-shaped relationship, where both too little and too much sleep raise all-cause and cardiovascular mortality risk compared to moderate durations. Sleep quality's evidence base is honestly much thinner and more scattered — many of the available studies address tangential subjects like loneliness or herbal supplements rather than direct tests of quality-improving interventions, with only one randomized trial (combining exercise and sleep hygiene) offering direct support. That doesn't mean quality doesn't matter — it's just far less rigorously studied on its own than duration is, so duration is currently the better-evidenced lever to focus on.