Conserving energy is probably the most important requirement in wireless sensor networks. In TSCH, this goal is obtained by subdividing time into slots, and by switching the communication interface of Internet of Things devices (frequently referred to as motes) off when, at any given time, neither transmissions nor receptions are scheduled for them. Nevertheless, in this kind of networks a considerable amount of energy may still be wasted due to idle listening. This occurs every time a cell is scheduled for frame reception but no transmissions are performed in the related slot and channel. In this paper, Proactive Reduction of Idle Listening (PRIL) techniques are introduced, which aim at lowering the energy wasted because of the above phenomenon. In particular, here we focus on a simplified mechanism that only considers the first hop (PRIL-F). A relevant feature of this kind of techniques is that they cannot worsen performance in any way. On the contrary, in those cases where they can be applied, they may only bring benefits. Results obtained through a simulation campaign show a tangible reduction in energy consumption, especially for periodic traffic generation, in application contexts based on either a star topology (wireless sensor and actuator networks) or a two-level topology (wireless sensor networks).