rogress in medicine has extensive relation with the other fields of science and technology. The through effect of the development of various sciences on human’s life and presenting health care services shows this relation. Emersion of body Sensor Network is one of these advances. Wireless body sensor networks will revolutionize health care services by remote, continuous and non-invasive monitoring. Hence it is expected that preventing and early detecting of disease will be added to the current healthcare systems which are mostly based on disease management and not on health management. In recent decades, health care services have been developed impressively. Due to the raising of the costs of health care and growth in life expectancy, Wireless Sensor Networks are offering the next evolution in biometrics and healthcare monitoring applications. By wireless body sensor network (W) system, a hospital can monitor physiology signals for 24 h without disturbing the patient. According to the attempts for convenience of users in all parts of life, the patient’s vital signs can be observed out of hospital environment. The main purpose of Body Sensor Networks is to transmit health information of users to medical center or physician in real time without any inconvenience for the users. These sensors can measure different vital physiological signs of patients like heartbeat, blood pressure, body temperature, pulse oxygen and other similar parameters. Health data is transmitted to medical center via wired or wireless connections. This research introduces medical sensor networks and its constraints, meanwhile discusses the requirements of these networks and proposes a solution for power reduction as the main concern in Body Sensor Networks. A Medium Access Control (MAC) protocol is presented and designed based on sample rate of each sensor and the number of bit per samples. This protocol tries to schedule sensors with less active time and more sleep time. This scheduling meets real time requirements and decreases active period, so inactive period will be lengthened, therefore power consumption will be reduced. By applying the proposed method, flexibility and expandability of sensor network will be increased. More over this research focuses on reducing control messages based on type of physiological signals and their continuity nature. Due to the slow nature variation of vital signs, there is no need to receive an Ack/Nack message for each packet. A window is defined regard to the least requirement of any biological signals that only one control message is sent for each window. The simulation shows the efficiency of this proposed method on the body sensor networks. Keywords Body sensor network, power consumption, MAC protocol, Duty cycle