When it comes to overwintering our prickly and succulent friends then the key is to keep them cool and dry, this is important as this will discourage them to continue growing during the minimal daylight hours and preventing stretched out etiolated growth and the cool temperatures will encourage them to rest for the Winter and then in the Spring they are more likely to produce flowers if they have had a Winter rest period. The only exceptions when it comes to keeping them dry is with the Autumn and Winter growing succulents such as the Mesembs, e.g Lithops and Conophytums, and also the Epiphyllum cacti that do not like the soil medium to be kept totally dry for long periods and most of the Epiphytes such as Schlumbergera and Rhipsalis are Winter flowering plants.
All the Desert cacti and succulents appreciate a dry and cool room to overwinter in that ideally receives some sunshine during the day, but if the only position you have to overwinter them is a bright but sunless north facing window then they will not come to any harm as long as they can be placed back into a sunnier position again from Spring onwards when they come back into their active growth again.
Overwinter the Desert cacti and other succulents at an ideal temperature no lower than 5-7c / 41- 44F but many growers have great success with overwintering their plants lower than this at 4c /39F but as I have no experience of overwintering them lower than 4c/ 39F I can’t recommend this from my own experience.
Melocacti, Uebelmannia, Discocacti and some other succulents are not as cold hardy as the Desert types and would recommened overwintering them at no lower temperature than 15c / 55F
Epiphytic cacti do not have a winter rest like the Desert types and ideally temperatures should drop no lower than 8-10c for these cacti during the Winter.
Check out my video below that I made for my you tube channel on How to overwinter you cacti and succulents where I talk about this topic in a lot more detail:
Cacti and succulent seedlings will need to be overwintered very differently during their first winter and do not rest over the winter months like mature cacti and succulents do, the different requirements with over wintering them all depends on whether you are growing them in a propagator with heat and grow lights or whether you are growing them in the baggies or without the baggies. Seedlings need to be kept warmer and also slightly moist over the Winter to stop the delicate root hairs from drying out, and it can be tricky trying to keep them moist without encouraging them to rot and to grow etioliated over the Winter if growing without special grow lamps.
I have made a video below where I explain in a lot more detail about How to overwinter your Cacti and succulent seedlings during their first year :