BLUE TONGUE SKINKS

Since I’m in the northern hemisphere, I’m wondering some things about brumation. My NSW locality eastern blue tongue’s been sleeping a lot etc. I’d like to try and actually brumate but the problem is that I won’t be able to get my room consistently lower than 20 degrees celsius. 18 degrees celsius at the lowest at night. I know NSW easterns especially get quite a bit lower temperatures than this. Will this hinder brumation or can I just turn the heatlamp off and let him sleep?