Thermal mass in summer
On warm summer days, walls and floors with thermal mass will steadily absorb heat at their surface, and store it until exposed to the cooler air of the evening/night. At this point, heat will begin to migrate back to the surface and be released. In this way, heat moves in a wave-like motion, alternately being absorbed and released in response to the change in day and night-time conditions. This ability to respond naturally to changing conditions helps stabilise the internal temperature and provides a largely self-regulating environment, reducing the risk of overheating and the need for mechanical cooling.
Graphic from Thermal Mass Explained. Download for more details.