Lighting energy consumption accounts for a large portion of electricity use in building and the exact amount of energy use is dependent on how occupants use lighting. This study aims to reveal visual environmental conditions, occupancy patterns, and their implications for the occupant use of lighting. The field measurement of four offices in Yongin was carried out from 1 June to 31 August 2010. External and indoor illuminances, and the use of lighting and entrance doors were monitored at ten minute intervals. The results from the field survey reveal illuminance on workplane, the start and end daily occupancy, and lighting use patterns by occupants. This study demonstrates that the lighting use pattern in the monitored offices is not statistically related to external illuminance, but has a close link with the start of daily occupancy. The lighting was turned on more than 93% the occupied period. We investigated the potential effects of different occupancy schedules on cooling and lighting energy consumption in summer using EnergyPlus. Increase in cooling and lighting energy demands due to extended occupied is discussed. Therefore, occupied period has a significant impact in the analysis of building energy consumption, and is an important factor to be considered in the energy simulation.