What I want it to do, is to sleep the entire time if the acceleration signal does not exceed a specific value.
How will you know that the acceleration value, along some axis, has exceeded some value, while you are sleeping? Unless you can program the IMU to output a digital signal when that happens?
You'll need to read the datasheet to see if you can do that.
- attachInterrupt(0,wake,RISING) calls a function "wake" that wakes the system once the input of 0 is rising
- as soon as the RISING state is finished, everything retuns to sleep by continuing loop()
If you can make the device output a suitable signal, the "rising state is finished" as soon as the signal triggers the interrupt handler. You do NOT want to immediately go back to sleep. You want to pay attention of the value(s) from the IMU and only go back to sleep when the output(s) from the IMU indicate that it is stopped.