This paper presents a new energy-efficient system by reducing beacon listening for periodic vital sign monitoring in wireless body area networks (WBANs). The node can reduce energy consumption by not waking up to listen to an unnecessary beacon when the data of the beacon frame rarely change. In proposed scheme, the node decides whether it wakes up or not for beacon listening according by previous received acknowledge (ACK) frame from the hub. The system is compatible with the IEEE 802.15 TG6 draft, which attracts attention as WBAN protocol. The results show that proposed scheme saves the power consumption to 14.7% when there is no network transition and 13.2% when network transition occurs at average of 2 seconds compared with that of the existing scheme.