A widespread approach to extend lifetime of battery-powered wireless sensor nodes is duty-cycling, which consists in periodically switching on and off node transceiver. However, energy waste in idle listening periods is still a bottleneck. These periods can be completely removed using emerging ultra-low power wake-up receivers, which continuously listen to the channel with negligible power consumption. In this paper, an asynchronous medium access control protocol is proposed for data gathering in a star network topology. The protocol exploits state-of-the-art wake-up receivers to minimize the energy required to transmit a packet and to make collisions impossible. The proposed approach has been implemented on a real hardware platform and tested in-field. Experimental results demonstrate the benefits of the proposed approach in terms of energy efficiency, power consumption and throughput, which can be up to more than two times higher compared to traditional schemes.