A radiation-based timer for use in an implantable medical device (IMD) includes a radiation source and a radiation detection circuit. The radiation source emits radiation particles during a process referred to as radioactive decay. The radiation detection circuit detects the radiation particles emitted during the decay process and tracks the number of radiation particles detected. When the number of radiation particles detected reaches a threshold value, a timer signal is generated. In this manner, the radiation-based timer generates a timer signal as a function of the radioactive decay of the radiation source. The timer signal may be used by one or more components of the IMD for any of a number of functions, including as a wakeup trigger for a communications and/or a sensor event.