They have heaters on them, so I don't notice any significant change in temp (all set to 70). That's my whole point. If you're creating an utterly artificial world in terms of heat and light cycles, winter shouldn't mean anything to your shrimp. It's like Groundhog Day inside the tank. The temp flux in the tanks is never more than a couple of degrees as the heater and lights cycle on and off.
Or maybe I'm not understanding the conditions other people keep their shrimp under?