If it is 40 right now, it will be in the 20's in there pretty soon! I am in Michigan and my shop is pretty big and super insulated. Because of this, my shop is still holding at 51 with no heat on overnight. As the ground freezes and creeps under the shop, the temps will start to fall. You should keep the temperature in the barn/shop, assuming it has a cement floor, so that they do not go below around 40 degrees. If you let the ground under the shop freeze and then you keep coming in the shop and heating it up, that's not good for the floor...the evidence is very clear if you look at the cracked concrete 2 foot out from my shop. That would be like a frost line right there, so when it snows, the snow melts all around my shop up to that point. I wouldn't want that effect to come closer or into my shop. Another thing too is when the temp really starts dipping down in the thirties you subject yourself to rust problems. The tools will draw some heat out of the ground and the cold air in the room will condensate on them. You should at least put a heater out there on low just to keep the temp close to 40 or above for tools sake. I have about $20,000 in tools and wouldn't sacrifice those to rust just to save heating money. That might sound like a lot of tools, but it's far from it. For that kind of money you are only talking about a half dozen pieces of equipment and tons of little tools like chisels, wrenches, screw drivers, cutter, drill bits..all that stuff will rust too and it's the little things that add up to far more money than one might realize.
To do regular woodworking, you can cut wood and sand wood at 40 degrees without issues. You just can't glue or finish it in the shop. Obviously, I recommend you come up with some heat.