The bees should stay up in the winter cluster. All the bees cluster round the queen and up in the top of the hive in winter. If the hive's interior is dry and it has an entrance reducer, and you don't get extreme cold, like under 0F, and they have adequate honey/pollen reserves, they should be fine. At night you can look in through the entrance with a flashlight and see how many dead bees are there. If it's a few, even a couple dozen you should be OK, if there is a great heap, you may need new bees this year. In a cold snap the housekeeping bees won't kick the dead ones out, when it warms up a little they will. If you are really worried, try putting a tarp over the hive, but be sure the entrance is uncovered (entrance reducer should be kept in place) so if it warms up overnight, and the bees want to forage next morning, they can.