I agree with jremington. Remember that human time and MCU time are different. An MCU can be asleep 90% of the time and still respond in less than a second. Or if timing is that critical then use a micropower circuit to detect the event and wake the MCU when it happens.
In case you didn’t get the other hint: If you would tell us more about what you are trying to do, we could be more helpful with our suggestions.