Natural and mixed mode ventilation systems are now often incorporated in the sustainable design of buildings in order to meet the increasingly stringent energy performance standards. An area of significant interest is the integration of these systems with atria as their large glazed spaces--a widespread feature in modern building design--have an untapped potential to enhance natural/hybrid ventilation.  This study presents experimental and complementary simulation results based on the long-term monitoring performed in an institutional building. The objective is to evaluate the performance of the building's natural ventilation system, examine its impact on the indoor conditions and assess its night cooling potential.  Long-term monitoring consisted of temperature and airflow measurements at the top atrium of the building, along with pressure difference and airflow measurements at the exterior air inlet grilles in the upper building levels. Experiments also included night-time ventilation rates and slab temperature measurements at lower building levels. A mathematical model was developed to estimate the cooling storage capacity of the slab.  The natural ventilation system at its present operation is able to cover only part of the building's cooling requirements. The application of modified ventilation strategies at night, making use of the building's thermal mass and cool outdoor air, holds a significant cooling potential. In order to achieve optimal performance and to avoid occupants' discomfort due to overcooling, a predictive control strategy should be implemented in the building. These operating strategies can be generalized for similar hybrid ventilation systems in buildings with high thermal mass.