A friend of mine owns a restaurant and insists that it wastes more energy (gas) to keep the place heated at night, then to turn off the heat (totally), and restart the furnaces in the morning. It gets quite cold in southern Ohio at night, and the resaurant dining room cools to about 60ish degrees by the morning. I disagree. I am looking for a site (or cite 
) that proves that I am correct. He won’t even agree that turning the thermostat down to 65 or so overnight will save energy. My Google-fu has failed me. Anyone got any ideas?
This sort of question seems to come up frequently (most recently with regard to water heaters, as I recall).
In general terms, your friend is correct: the amount of energy lost from a building decreases when the temperature difference between inside and outside is smaller.
I did have a cite for this a while back, but basically he is correct. The energy savings can be estimated at the time at the steady state lower temp. though some energy savings can be realized by having the furnace off for an hour then have it heat back up since during the time the place is at a lower temp you will have less heat transfer to the outside.
With the furnace turned totally off he is risking bursting the pipes if it gets cold enough, but again he is saving energy, as less heat is transfered to the outside.
Ideally a structure should be heated to at least 50F to prevent other problems such as cracks in drywall from tempature swings.
Your friend is correct. If you search in here you will find numerous threads where this is discussed from many different angles.
There are some links, and discussion, in this recent thread.
http://boards.straightdope.com/sdmb/showthread.php?t=339135&highlight=thermostat