The Economic Consequences of Increasing Sleep Among the Urban Poor
Those in the study were low-income adults in Chennai, India who sleep only 5.5 hours per night on average despite spending 8 hours in bed.The key takeaway was:   > increased nighttime sleep had no detectable effects on cognition, productivity, decision-making, or well-being, and led to small decreases in labor supply.

I find the result surprising. When I get under 6 hours I see a clear drop off in performance.