But it wasn’t completely clear that the app’s sound cues led to that increase. “It could be that just focusing on lucid dreaming for a week or expectations or something was responsible,” Konkoly says. So the team ran another experiment with 112 people.

Everyone got lucidity-triggering sounds from training while they slept the first night. But on the second night, the app — unbeknownst to the users — switched things up. Only 40 people heard sounds from training while they slept. Another 35 got sounds they had not practiced linking to lucidity. The final 37 heard no sounds.

The first night, 17 percent of participants reported lucid dreams. The second night, people who heard the sounds from training kept up that rate of lucid dreaming. But only 5 percent of the people in the other two groups had lucid dreams — hinting that the real sound cues were indeed behind the app’s effectiveness.