The energy saving contributions of thermal mass are well known but not always implemented in an ideal way. There are many examples of buildings designed and built to optimize and take advantage of all the energy savings and comfort that thermal mass has to offer. Some function well, while others are complete failures. There are plenty of examples of passive solar homes built in the 1970s that never came close to living up to predicted performance.
To work correctly in a building, thermal mass must take into account temperature changes on the exterior, as well as temperature changes in the interior. Mass absorbs heat when heat is present and releases the absorbed heat when the temperature drops. In a warm climate, a building with exposed concrete enclosure elements does a good job absorbing the heat of the sun during the day, helping to keep the building cool. When the temperature drops at night, this heat will continue radiating into the space, causing discomfort to the occupants.