This paper presents a method for realizing energy neutral operation on energy harvesting wireless sensor nodes (WSN) and its implementation, regarding that the available environmental energy is unpredictable and changes over time. The method utilizes adaptive duty cycling which provides energy-neutral operation according to the energy available in the environment and the instantaneous energy state of the node through an energy management circuit. The proposed method is implemented using a MicaZ mote as the WSN and two different vibration-based harvesters: piezoelectric and electromagnetic. The node incorporating a piezoelectric harvester, operates for only 130.5 s with a fixed duty-cycle of 0.21%, and requires an inactive time of 93.5 s for charging. On the other hand, with the proposed strategy, the node achieves energy-neutral operation by self-adjusting to 0.17% duty-cycle. Energy-neutral operation is also demonstrated by incorporating an electromagnetic energy harvester attached to the wrist of a runner: When no energy is available for harvesting, the proposed strategy shows about 64% increment in lifetime before going to sleep mode. These demonstrate that the proposed energy management policy proves to achieve energy-neutral operation in an efficient way.