We brumate in a dark room, no light during the day only daylight once a week when we clean the tubs (they still drink and pee even when not eating) and give clean water weekly. They are not fed during brumation and not for a month before the temperature is dropped - to give them a chance to fully empty the gut. Undigested food at lower body temperatures is a health risk.
Brumation temps for corns are about 55 degrees - they can go lower or higher for short periods of time but they stay pretty well rested when around 55 degrees. Higher temperatures (60-70 for prolonged periods) have been associated with increased risk of respiratory infection. Anything below 45 degrees is getting pretty risky, in my opinion - we adjust the room temps when it threatens to drop below 50 at night.
Be aware that they may be a bit nervous and more likely to tail rattle when disturbed during brumation - we provide a hide area for them and just lift it to see that they are ok, but do not handle them during brumation. Any animal that we suspect may not be normal is brought out of brumation early to address the concern.
mary v.