In wireless sensor networks, much energy is wasted during connecting and control processes. This is particularly true for the neighborhood discovery process carried out in Bluetooth Low Energy (BLE) before the communications between devices. However, the fixed duration of this process has a critical energy cost when the number of neighbors is low or when only a few neighbors are required by the application. In order to address this issue, a novel protocol called WUBBLE is proposed to leverage wake-up radio technology to start and end the discovery process. Wake-up radio enables additional communications between devices with an ultra-low energy overhead. WUBBLE is validated by combining analytical analysis and experimental measurements, and results show that half of the energy could be gained to discover 90% of the neighbors.