From this article:

“Being an academic neuroscientist, I didn’t limit my research to the Google perspective, let’s call it. I leveraged an advantage that few consumers have—access to peer-reviewed research and the training to critically interpret it. […] With help from colleagues, I sifted through thousands of academic papers that investigated neurofeedback. Our main finding? At the time of our research, only nine neurofeedback studies had used an adequate control group and a double-blind design (where neither the experimenters nor the participants know who receives the real treatment). Many of the studies showed that neurofeedback could improve some behaviors, but no more than if participants underwent the same procedure and received false feedback pre-recorded from another participant. Changes in behavior relied purely on placebo effects.”

“Since then, researchers have conducted two of the most rigorous neurofeedback experiments to date. They showed that neurofeedback can both substantially improve attention and also make us think we sleep better, but not actually improve our sleep. Again, false feedback worked just as well as real feedback. In other words, it doesn’t matter if you watch your own brain, or if you watch a recording of Jim’s brain from last week, you’ll improve all the same. The benefits seem to stem from being highly motivated and exerting effort, rather than from altering brainwaves.”