With SPOTIFY I suspect that they are deliberately stopping the stream after assuming that you are playing to the plants and SPOTIFY wants to conserve bandwidth. Another possibility is that your ISP is rebooting the router overnight. My ISP will often reboot the Gateway between 2:00am and 3:00am on Wednesday. My Internet connection and WiFi will go down for a few minutes during that reboot.
One technique to work around SPOTIFY dumps and Internet outages would be to store the noise tracks locally on a device that does not go to sleep or lose it’s network path if the router is rebooted. A technique to end run a SPOTIFY dump (if this is the issue) might be to make up a random order looping SONOS playlist of several different noise tracks. To SPOTIFY this might appear as a human interacting with the service and they will not dump the stream. Yet another trick to fool SPOTIFY and potentially work around a router reboot would be to setup some alarms, spaced through the night, that will start the noise tracks fresh. If the stream is stopped for some reason, the next alarm will restart the stream. Note, however, there is risk that if the alarm track cannot be started for some reason, say because the router is rebooting, the alarm will start to ‘Gong’.
With regard to random playlists: A SONOS playlist is stored on your system. A SONOS playlist is only randomized once. Therefore a “randomized” playlist will always play in the same random order. If you make a change to the playlist, it will be freshly randomized. I don’t know if SPOTIFY is smart enough to figure this out. You can also setup SPOTIFY playlists that are stored on their server. They might be smart enough to figure out that you are repeatedly cycling through that playlist and terminate.
The above are somewhat esoteric possibilities. You could also have some sort of transient issue on your network.
Keep a log if events because you may be able to notice a pattern. You may not notice a brief outage while using email or a web browser, but music and streaming video are more sensitive to service interruptions.
Within 10 minutes of an outage, submit a diagnostic and post the confirmation number. SONOS staff will often comment on a diagnostic here or you can contact phone support.
I switched to a 90 min noise track from Spotify and haven't had a problem for a few days.