Can lights are a major source of air leakage in the home. The fixture along with the hole create a large gap for conditioned air to escape from the home. During the summer cooling season can lights allow hot attic air to travel into the home. Signs of air leaks in the attic is dark stain marks in the insulation. As the air travels in/out of the home it carrier dust and other allergens. The fiberglass insulation traps some of these particles creating the dark marks. With proper air sealing then insulation in the attic these leaks are stopped creating more comfortable rooms and lower energy bills. To determine how many air leaks are in your home and energy audit needs to be completed with a blower door.