Wireless sensor networks are one of the key technologies to ensure ubiquitous connectivity for the internet of future where billions of things must be connected. As sensor nodes are generally energy-constrained, the critical issue to face is how to reduce energy consumption so that the network life is prolonged. Time Division Multiple Access based protocols are inherently energy-efficient since nodes are awake only during their slots and sleep the rest of the time. To our knowledge, few are the works that have investigated the impact of a dynamic TDMA frame size on the network energy consumption. In this paper, we propose a new energy efficient MAC protocol for wireless sensor networks, called Sensor One shot Slot TDMA-based Reservation (S-OSTR). S-OSTR brings the idea of a dynamic active period, since it deploys a dynamic frame size that is built slot-by-slot according to nodes arrival to the network leading to a shorter achieved frame size. The short frame size combined with an appropriate spatial reuse scheme contributes to significant save of energy. The S-OSTR energy efficiency has been approved by a set of simulations comparing our proposal with SERENA [1].