Evaluating Investigations: Practice

Two studies test a sleep-tracking headband. Study A: 500 randomly selected adults, half wear the real headband and half wear a fake version, neither group knows which they have, and an independent lab repeats the study with similar results, finding sleep improves slightly. Study B: run by the headband's maker with 8 volunteers who all know they have the real headband, finding sleep improves a lot. Which study is the better basis for a conclusion, and why?