This article presents SCSP (for ``Sleep Collect and Send Protocol'') for wireless sensor networks, a network-MAC cross layer design that resolves the inherent conflict between energy efficiency and throughput. The protocol uses in its MAC layer a new paradigm that we call ``sleep, collect and send''. The idea of SCSP is that a router sleeps for a given amount of time, wakes up and collects data from its children and other routers and then send them into a burst during a period of time that we call transmission period. In its network layer, the protocol uses a hierarchical tree structure as network architecture and a tree routing protocol. SCSP does not require synchronization between routers and dynamically calculates the sleep and collect periods according to the amount of incoming traffic. The protocol is implemented and simulated in OPNET simulator.