No. But if you disable the Ungroup on Autoplay feature in the Sonos app, it should lessen the amount of times you need to regroup the speakers.

The group that I’m doing in the evening are again ungroupped in the morning. Most likely because WiFi is off during the night. How this be somehow preserved to avoid every-morning grouping of the rooms?

If you turn off the network Sonos is going to un-group.

You could create a Group using your controller to make regrouping a lot easier.

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

Save groups
You can save a group of rooms that you frequently group together and create a shortcut in the Groups menu. Saving groups is only available in the Sonos S2 app for iOS or Android. From the 

 Settings tab, tap System > Groups to manage your saved groups. Follow the steps below to create, edit, delete, or play to a saved group.

Hi. System > Groups is available only once you have 3 or more rooms defined. I’m having two only (Arc TV set and Move). I would like to have these 2 working together most of the time. Unfortunatelly every morning I need to group these 2 rooms together - as said most likely because WiFi was off during the night.

Why are you turning off wifi at night?  I’m curious.

You might want to look into setting a routine or alarm that will play a radio station, playlist, or silent track to the group every  morning to regroup.

I actually have 3 rooms group and play music every morning, but I do it via a 3rd part smart control app, but using the Sonos alarms directly.  Not sure if it can be done that way.  I’m sure it could be done using Alexa groups and routines though.

WiFi is off to reduce the electro-smog while not needed.

Alarm from what I see can be setup for individual rooms - cannot turn it for more than one room at once. There is an option to include groupped rooms, but at the moment when the alarm would ring, they are already ungroupped.

You could wire your Sonos with Ethernet and then turn off the internal radios to further reduce the electro-smog.

If you are turning off the whole router, not just the WiFi you’d want to hook the Sonos to a cheap ($20) Ethernet switch and then that to the router.

That will probably keep them grouped.

If you’re concerned about electro fog, I’d certainly suggest choosing a speaker that isn’t designed for use on a WiFi signal. Look for anything wired, not purpose built for WiFi connection.