Sinking costs in the production of low power silicon technologies and intelligent energy saving strategies have lead to wireless communication for sensor networks becoming highly desirable. In the different fields of application for such networks, which include building automation, car automation or container tracking, there is a common need to have nodes which are completely self-sufficient and last several years on battery operation. A long life is usually realized by scheduling periodical power-ups and power-downs of the node and synchronizing the communications. This work proposes a particular radio receiver called Wakeup Receiver, which allows incoming communication without schedule, removing the constraint of complicated synchronization protocols. This design allows for powering up the nodes on-demand, resulting in much more efficient energy saving strategies and communications. The impact on the overall network energy is also analyzed using a proof of concept custom integrated circuit. Finally, the implementation of the receiver is presented, which performs within the range of expected power consumption. With the proposed solution, sensor networks will achieve efficiency in energy consumption and communication, such that their implementation and utilization will bring great advantages to their application fields.