This is a question of much debate in the poultry industry. Many chicken breeds are naturally quite cold hardy but it always important to research your breed to ensure that you are aware if they have any specific needs. Even in cold areas, many chicken owners feel that providing artificial heat alters a chickens natural adaptability. For those that do choose to make their flock more comfortable and continue to be winter layers there are various heating options available that we have researched.
Heat lamps are the long term standard in the industry but many fear the risk of fire. Based on our experience, we have found that if you choose to use heat lamps it is important to ensure that they are high enough that the chickens are not hitting them or knocking into them. In addition, using the metal casing and a higher quality bulb help as well. There are two options for the heat bulbs, clear or red. We learned very quickly that clear wasn't working for our flock. These bulbs were bright and our ladies and gent were partying 24/7. We switched to red bulbs and they now sleep again.
A newer option that has become very popular is a cozy coop. This is a heat releasing mat with little fire risk but note that they release limited heat and heat can only be felt right in front of the mat. I think that the cozy coop may be a great solution with a small coop with only a hand full of chickens but was not a feasible solution for us.