We have developed a new battery-operated sensor node based on ZigBee wireless network in order to track people's location even with a poor electrical infrastructure. For a sensor network consisting of such kind of sensor nodes, we have designed a flexible sleep control protocol which fulfills low-battery consumption as well as high-accuracy time synchronization between these nodes. We have deployed a sensor network system composed of more than 40 sensor nodes in a hospital floor for tracking more than fifteen nurses simultaneously. Experimental results show that the system can operate around 27 days using four AA size NiMH batteries and can achieve 90% accuracy in the location estimation.