Are they fully feathered? Everywhere, under the wings, belly etc. If they are completely feathered, I would wean them off of the light. Do you know what the temp is where they are at the coldest point of the night? Because it is so cold where you are I would take it a little slowly as they are not used to those temps.
I always wean my hatchlings off of heat/light. Get a timer, they are under $10, get one that you can set ALOT of changes. I have one that has little buttons that I can push down every 15 minutes for 24 hours. Start by turning it off during the day completely if you haven't. In your temps, I'd probably start with 2 days of off for 30 minutes, on for an hour or two, off 30 min, etc during the coldest part of the night. You could do an hour off at a time for less cold times. I'd do 2 days, then extend the times for a few days until the light is off.
This may seem exsessive but I had some losses when I just turned off the light and they panicked.