Sound for focus is not sound for sleep
The same recording that puts you under at midnight will fog your afternoon.
Work sound should be even and slightly impersonal: steady rain, a café at low volume, a fan. Nothing with a voice, nothing with a shape — the moment there is a story, part of your attention goes to follow it.
Sleep sound can afford to be personal, close, and slow. Voices are fine there, because you are not trying to keep any attention in reserve.
Mixing the two is the common mistake. People put on a whispered recording to concentrate, drift, and conclude that sound does not work for them.