Medium Access Control (MAC) sublayer solutions based on Adaptive Time Division Multiple Access (TDMA) have been proposed to satisfy temporal constraints and improve the reliability and efficiency of Wireless Body Area Networks (WBAN) subject to unexpected communication conditions, such as dynamic message traffic and communication interferences. However, many of these solutions do not efficiently use communication channels, allocating time slots to devices that do not have data to transmit. It can be a problem because wasting time slots reduce the ability of the WBAN to recover from message loss under unexpected communication conditions. This paper introduces the THeTA, an adaptive TDMA policy for WBANs that meet time constraints and enhance the reliability and efficiency of WBANs under unexpected communication conditions, observing a better usage of the communication channels. To improve message delivery and not compromise temporal constraints, our approach allocates unused time slots from devices with low bandwidth demands to devices with high transmission demands. Compared to IEEE 802.15.4, IEEE 802.15.6, and other adaptive approaches, it was observed that the proposed approach has higher reliability, reducing message loss and maintaining low power consumption and low latency in the evaluated scenarios.