This paper presents a new type of wake-up receiver, called a clock-harvesting receiver, which extracts a 21 Hz clock embedded within the GSM standard for the wake-up of a wireless sensor network. The receiver was fabricated in 0.13 μm CMOS and harvests a 21 Hz clock from the 1900-MHz GSM band. It was designed for heavily duty-cycled operation to reduce the energy required for synchronization of a network. In active mode, the receiver achieves a sensitivity of -87 dBm with 57 μs of jitter while consuming 126 μW. In sleep mode, the power consumption is 81 pW. Experiments conducted in the lab verify the functionality of the receiver with a real local GSM cellular signal.