I've wondered about that. I think that is technically true, but to a degree that is not very significant. My understanding is that your caloric requirements will go up VERY slightly in colder temperatures, unless you make yourself so cold that you constantly shiver. That's not very healthy or pleasant. I'd rather just eat less than sit and shiver.

What indicates there's a significant savings here? How many calories will you burn by dropping your thermostat from 70 to 64? I think it will probably be only perhaps a few calories per day.