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). However, because both sensor nodes and CubeSats operate on very limited energy supply and storage, the efficient management of power-hungry communication sub-systems is essential. Thus, specific Medium Access Control (MAC) protocols are needed to ensure the minimal overhead while considering DtS-IoT scalability and channel dynamics. In this work, we contribute with REserve-and-Send Sift-IoT (RESS-IoT), a scalable and energy-efficient DtS-IoT MAC protocol, combining the LoRa physical layer with a novel link scheduling approach. Extensive simulations demonstrate that our solution provides up to four times the energy savings in the satellite and up to seven times the energy savings at the sensor node on the ground with respect to state-of-the-art IoT protocols.