If it is in the 50s and 60s at night I would still keep the heat lamp on but I would raise it up a good 3 feet or more. Just keep raising it up.
I have 4 week olds and 6 week olds and I left the heat lamp on for the 4 week olds chicks and I noticed that from time to time the 6 week olds would go lay under it and stretch out for awhille. I didnt turn the heat lamp off until it was in the 80's during the day and 60's at night.
I noticed that when I had turned their heat lamp off that they had been sleeping huddled together in a tight ball, scared to death. I plugged a little night light in and placed each one on the roost. I think I will just keep a night light in there, i dont think they can see at night.