In this paper, we design an energy efficient MAC protocol for mobile sensor networks. In our MAC protocol, to reduce idle listening, each node has its listen-sleep schedule. A subset of nodes that follow the same schedule forms a virtual cluster. The network consists of multiple virtual clusters and nodes in one virtual cluster may follow a different listen-sleep schedule from that of nodes in another virtual cluster. Hence, to support a mobile sensor node that moves into a new virtual cluster that follows a different listen-sleep schedule, it is necessary to have a fast and energy efficient listen-sleep schedule adaptation protocol. To this end, in our MAC protocol, we utilize schedule information on border nodes between virtual clusters. This allows us to implement fast and energy efficient listensleep schedule adaptation, which consists of two main functions: energy efficient secondary listen period and smart scheduling adaptation. Simulation results show that our protocol can provide fast scheduling adaptation while achieving energy efficiency.