Instead of heat lamps you could use a ceramic heat emitter which will raise your temp throughout the entire cage and can be used 24/7 as they put off no light
http://www.amazon.ca/Zoo-Med-ReptiCa.../dp/B0002AQCPK

No matter what you go with it should be on a thermostat to regulate the temps. I use herpstats http://www.spyderrobotics.com which can be fairly pricey but work extremely well and last a long time

Another item which can be surprisingly helpful is an IR temp gun http://www.amazon.com/HDE-Non-Contac.../dp/B002YE3FS4
not only will it help you dial in exact temps throughout your cage, but also to get live feeders to switch.
One of my girls would not look twice at a room temp f/t rat but using a hair dryer and a temp gun I was able to get the rat to its normal body temp (37c) and she snapped it right up.