It depends on how cool it gets and what Hoyas you have, some like it or can tolerate it cooler than others. Hoyas will grow all year round if given enough light and warm enough temperatures and to do that I place most of my smaller plants and cuttings in a southern exposure and give them supplemental fluorescent light for about 12 hours a day. I have some larger Hoyas that are also in south facing windows without fluorescent lighting but they are still kept warm. As long as the day time temperatures are high enough during the winter months and the night time low temperature does not fall too far below the preferred range for the Hoyas you have they should just slow their growth but remain healthy during the winter as long as you adjust their watering schedule. If they are new cuttings I would recommend keeping them as warm and bright as possible and grouping them together maybe under a daylight compact fluorescent bulb so that they continue to grow for you over the winter and are just that much closer to blooming size come next year.

Mike