I like Binaural Beats for this, plenty of Apps on offer.
Started out with this one and hacked a bit on it, never published my version though: https://f-droid.org/en/packages/com.github.axet.binauralbeat...
Now i'm pretty content with this one: https://play.google.com/store/apps/details?id=com.project.rb...
And this tune, it now is on my phone and every PC thanks to NewPipe.