[tinco]

Why would it be unsafe to sleep? They are inside the house so they are protected against the elements.

I'm pretty sure sleeping in your car would be more dangerous just because of the discomfort leading to worse quality sleep and fatigue.

[tinco]

(Just to make it clear I didn't mean to downplay this. Obviously to survive in your house at -10c you do need preparations which I understand many people don't have not being used to these temperatures. It's easy to forget as someone who lives a bit further up north that it's not a given that people have thick air tight walls and heavy down filled blankets to help them through. So to answer my own question: It would be unsafe because they might not have the things they need to be safe.)