Hospitals, powerplants, chemical plant, generally any large installation have been dealing with critical issues during off-hours for centuries. The solution is an on-call rota. Labour law knows about this, and so do the unions.

Actually, the on-call rota is one of the things I ask about in interviews. If the interviewer doesn't know what to say or says they don't have one it's a showstopper. Operations are going to be a horrible mess, and it's not a place anyone would like to work at.

Man, I hope nuclear plant monitoring isn't done via email.

It is likely that there are exceptions.

More than 10 years ago, a coworker of mine traveled to Netherlands to set up some servers in a data center there. He was utterly amazed to see data center employees simply dropping what they were doing and leave work at 5pm sharp. My coworker needed help from them to set up something but had to wait.