I’m having the same trouble you are with waking it back up. In some of my sketches, I call the shutdown code within the loop and by the time it starts looping again, it wakes up. Other times it goes to sleep and can’t wake up. I’ve been looking for some alternate ways to go in a low power mode while still being able to monitor readings from the IMU, but I’m lost. Here’s the nRF52840 docs so you can have a look.