Power consumption is an essential challenge in medical application devices using WSN, where the patient who carries a battery power system. Thus, an energy-efficient monitoring system that can independently measure the vital signs of the patient is necessary. This study presents an energy-efficient wearable patient monitoring system (WPMS) to monitor the temperature, heart rate, and oxygen level in the blood (SpO2). A low power consumption ZigBee wireless protocol was interfaced with Arduino Pro Mini microcontroller based on Atmega 328P to alert caregivers in real-time during emergency cases when risks are observed in the patient's biomedical signs. A sleep/wake algorithm has been proposed and implemented inside the microcontroller to improve the power consumption of this wearable device. Results show that the power saving of 92.39% was achieved based on sleep/wake algorithm relative to the traditional wearable device (i.e., without sleep/wake algorithm). In addition, the battery life was extended to approximately 15 days relative to the traditional one-day operation. Comparison results disclosed that the WPMS outperform the power consumption of other studies in medical applications. Where the average current consumption of 6.13 mA is obtained based on the sleep/wake scheme. We can be concluded that the proposed system is energy-efficient, real-time monitoring, cost-effective, none-complex, comfortable due to it is dispense of wire connection, and easily implemented