Another possibility is to use an engine block heater - if the Altima provides for one.

A lot of Prius owners a using engine block heaters to do just that. Keep the car warm to improve fuel economy in cold weather.

But Toyota USA didn't provide much help. The Prius owners had to check with Canada to find out that the car did have provisions for an engine block heater. They then were able to order the heaters. (The heaters are made and shipped from Canada!)

And to top it off, the service manual doesn't show where the heater goes! People had to figure that one out too!