What are your night time temps? When I first got into chicks, I was advised to keep one area of the brooder at 90°F for the first week and to reduce the temp by 5° each week until they were at the ambient temperature. (Another area should be cooler so they can choose their area of comfort.) By five weeks they should be fully fledged and no longer need supplemental heat, especially if you have several chicks that can huddle together to keep each other warm. Now that we've raised several crops of chickens, we've learned to let the chicks tell us what they need. Loud peeping is a sign of distress. If they are cold they will huddle together (this is normal when they are sleeping anyway, but they will make calm, happy noises, like purring). If they are too hot they will hold their wings away from their bodies and pant, trying to get away from the heat source. Happy chicks are active chicks.