Wireless sensor networks (WSNs) are increasingly used in environmental monitoring applications. They are designed to operate for several months by featuring low activity cycles, in order to save energy. In this paper, we propose a MAC protocol for such WSNs with duty-cycles of 1%. Initially, nodes are activated randomly and independently, then they use the knowledge of previous successful frame exchanges to compute their next activation times. We study the choice of the history size, and we compare the performance of our protocol with other protocols from the literature. We show that with a limited history size of only six entries, we significantly improve the performance of existing protocols, while keeping the advantages of fully asynchronous protocols.