It takes time for the heat to travel through a substance that is heated on one side. Heat lag is the time that is needed for heat to travel through a substance on one side. The sun heats the outside wall of a building. However, several hours pass before this heat reaches the inner surfaces of the wall. In normal buildings this time varies between three to four hours. With well insulated or thick walls, the sun may be gone by the tune the heat soaks through.
The south wall receives sun all day, while the east and west walls receive sunlight only for a small period of time. However the south wall is not as strongly affected as the east and west walls. This is because the rays come in overhead. The heat lag can causes rooms to be heated even after the sun goes below the horizon and the outdoor temperature drops.