I'll add a bit to @kerkeslager response in stating that a placebo is something that works because you are told it will work. Your example of death metal would only be a placebo affect if someone told you that listening to death metal will help your sleep and then it did. If you are just a hard core rocker and got used to listening to death metal to sleep and now you find that without it you don't sleep as well, that isn't the same thing as a placebo affect.
But I am interested in what a death metal sleeping app would actually look like. Of course I would have to ask if it's needed in the market place (does it create value), as opposed to just playing my Demented Ted playlist.
> I'll add a bit to @kerkeslager response in stating that a placebo is something that works because you are told it will work
Yes, that was part of my initial point - to the extent that placebo exists, anything can be turned into a placebo with enough marketing. Avoiding blue light could help with sleep quality. Adding more blue light could help with sleep quality. Faith healing could help with sleep quality. Homeopathy, traditional Chinese medicine, ayurvedic medicine, leeching, blood-letting etc. If it only depends on you believing in it, but it does help, I would recommend the cheapest possible remedy that you can bring yourself to trust, and that doesn't have a proven anti-effect (coffee therapy for curing insomnia may not be an effective treatment).
I also detailed some skepticism of the placebo effect in another response here.
But I am interested in what a death metal sleeping app would actually look like. Of course I would have to ask if it's needed in the market place (does it create value), as opposed to just playing my Demented Ted playlist.