Auto Switch off every day?

You mention you have an 'automated smart house', there are many integrations across ST, HA, Ikea and other smart home options as well as Alexa and others that can achieve what you are wanting.

Nothing native to Sonos at present though...

What ‘smart house’ automation are you using? Possibly create a routine to pause all players and schedule at midnight, as above, most have interfaces with Sonos.

If you use Apple devices, create an iOS shortcut, and automation schedule. Can also be triggered when you leave property etc.

Remember to disable the automation if you are having a party that goes beyond midnight though. 😀

If you are not inclined to work with some type of Home Assistant and assuming you are referring to one particular Sonos speaker you might consider a timer similar to the one shown below.  However, stopping a stream in that manner may be similar to an unplanned power outage. The question will be how efficiently will the speaker re-integrate itself upon power-up to your network without IP address conflicts which could be an issue. The timer pictured is from Amazon of which there are many choices.

You could schedule a short silent track to play every day at midnight. After the track finishes the players will go into standby.