I'm having the same issue with a 2 month old M1 MacBook Air.
It crash restarts after it has been in Sleep Mode for a few hours.
The rest of time it works ok, apart from one thing.
I've noticed occasional odd behaviour with Mail open.
Sometimes clicking on the Yellow "reduce to Dock" button does nothing.
I have to close Mail and then open it again to fix it.
The Mail thing seems to only happen when the MacBook Air has been on for a while, but hasn't moved into Sleep mode for hours.
None of this happened until I updated Big Sur to 11.3.1.
But, from what I've now read online, it seems I must have been lucky (before 11.3.1.
The crashes while in Sleep mode seems to be a well documented problem.
I've sent the crash reports to Apple.
I've tried safe mode, reinstalling 11.3.1.
I have NO third party Apps apart from Microsoft Office, HP printer stuff and ClamXAV.
Come on Apple, get it together please.