The Internet of Things (IoT) is enabling unprecedented applications based on the transport of small data volumes to and from constrained devices. When end-devices or sensor nodes are located in very remote zones with inaccessible topography, Direct-to-Satellite IoT (DtS-IoT) has been proposed as an appealing solution. In DtS-IoT, isolated sensor nodes can directly relay data to and from inexpensive nanosatellites (i.e., CubeSats). Because both sensor nodes and CubeSats operate on very limited energy supply and storage, the efficient management of power-hungry communication sub-systems is of the essence. Thus, specific Medium Access Control (MAC) protocols are needed to ensure the minimal overhead while considering the DtS-IoT scalability and channel dynamics. In this work, we contribute with REserveand-Send Sift-IoT (RESS-IoT), a scalable and energy-efficient DtS-IoT MAC protocol, combining LoRa physical layer with a novel link scheduling approach. Extensive simulations demonstrate that our solution provides up to four times energy savings in the satellite, and up to seven times on the sensor node on ground, with respect to state-of-the-art IoT protocols.INDEX TERMS Direct-to-Satellite IoT (DtS-IoT), Energy efficiency, LoRa, Medium access control (MAC), Nanosatellite