Low-power transmissions of sensor nodes are easily corrupted by interference generated by co-located wireless terminals that leading to packet losses might increase energy consumption and result in unreliable communications. Dynamic spectrum access mechanisms can mitigate these problems allowing cognitive sensor devices to sense the spectrum and access the wireless medium in an opportunistic way. With this respect, energy efficient algorithms for spectrum sensing have to be designed in order to meet the power constraints of wireless sensor networks. In this paper we consider an energy constrained system comprising two sensor nodes that avoid interference by exploiting spectrum holes in the time domain. We design the algorithm used for spectrum sensing so as to minimize the average energy required for the successful delivery of a packet. While carrying our this task we adopt a cross layer approach that accounts for the average channel occupancy and the power of interfering transmissions at the physical layer as well as for the size of packets used by sensors at the transport layer. Our results show that accounting for the short length of packets commonly used in sensor networks can significantly improve energy efficiency leading to gains of up to 50% if compared to other spectrum sensing algorithms envisaged in the literature.