A system and method for waking up an implantable medical device ("IMD") from a sleep state in which power consumption by the IMD is essentially zero. The IMD may be adapted to perform one or more designated measurement and/or therapeutic functions. In one embodiment, the IMD includes a wake-up sensor that is adapted to sense the presence or absence of a wake-up field generated by another IMD or an external device. The wake-up field may, in some embodiments, be an electromagnetic field, a magnetic field, or a physiologically sub-threshold excitation current (i.e., E-field). Upon sensing by the wake-up sensor of the wake-up field, other components of the IMD, which may include a controller, a sensing and/or therapy module, and/or a communications module, are awakened to perform one or more designated functions.