Basic physics pretty much says that if the springs are colder they won't flex as much, but I wouldn't say they are brittle.
My bocks live in the porch which isn't actually attached to my house and stuff does freeze in there in winter (it was here when we bought the place and it is literally some bricks put on concrete infront of the house, there's an obvious gap between the bricks of the porch and those that make up the front wall of the house), so I usually bring them into the house to warm up by a radiator or something for a while before I take them out. If you keep them in the house all the time you can shove them on and go out bouncing straight away.