A wireless body area sensor network (WBASN) demands ultra low power and energy efficient protocols. Medium access control (MAC) layer plays a pivotal role for energy management in WBASN. Moreover, idle listening is the dominant energy waste in most of the MAC protocols. WBASN exhibits wide range of traffic variations based on different physiological data emanating from the monitored patient. For example, electrocardiogram data rate is multiple times more in comparison with body temperature rate. In this context, we propose a novel energy efficient traffic-aware dynamic (TAD) MAC protocol for WBASN. The protocol relies on dynamic adaptation of wake-up interval based on a traffic status register bank. The proposed technique allows the wake-up interval to converge to a steady state for fixed and variable traffic rates, which results in optimized energy consumption. A comparison with other energy efficient protocols for three different widely used radio chips i.e., cc2420, cc1000, and amis52100 is presented. The results show that TAD-MAC outperforms all the other protocols under fixed and variable traffic rates. Finally, life- time of a WBASN was estimated and found to be 3-6 times better than other protocols.