Update: This work-around didn't succeed...at all. While the test went just as I described, turning the Private Session on before sleeping and attempting to have it left on all the time Alexa played music did not. I don't know when the Private Session ended, but the desktop client is no longer showing the tiny lock on the profile nor checked as active under File. I will have to switch to Amazon tunes as the sole source of ambient noise. *grumble* Spotify, please, a little help...
I'm also regretting that my sleeping soundtrack has ruined my recommended new music. My most recent Discover Weekly isn't the terrific quirky music that felt so attuned to my tastes as to be uncanny. Instead, it's turned into the new age and nature sounding droney stuff chosen specifically to be unexciting and sleep-inducing.
However, I just tested something that *might* help. On my desktop PC, with the stand-alone Spotify client (not browser) where I normally listen to the "good stuff," I set the session to private. Then I went to the bedroom and asked Alexa to start the sleepy playlist, came back to the PC and checked--it was still showing the little lock on my profile and was apparently playing on the Echo while in private mode. That might not be how it's supposed to work, but appears to be what, in fact, happens.
I will continue to set it to private before going to bed (a bit of fuss to have to remember to do that) and hope that after a few weeks my Discover Weekly will recover to its former delightful new music and not the snooze-ic my sleep-aid playlist changed it into.
There really needs to be a bit more user control over what comprises the Discover Weekly list. I thought by not "liking" the sleep playlist tunes, the recommendations wouldn't be based on what was played for hours at night. But, nope, the most recent list is a deluge of calm and instrumental music that's well outside my typical preferences.
Also, ideally, Alexa could understand a simple request to turn on "Private Session" but when she is unable to simply "Like" a song, I'm not expecting that functionality anytime soon, but better integration with Spotify would be wonderfully helpful.