Ben.. It's not clear what your question is? I use dim=0 which allows my MCU to send updated values that get updated on the Nextion. So, when I send dim=50 (or some other value), the Nextion lights up with updated data. If you are trying to enable sleep to save power, then you may have to come up with another way. That said, dim=0 saves power by not lighting the TFT, and it allows the Nextion MCU to keep processing commands over serial.
Sleep mode (sleep=1, or sleep entered after no input via settings)
- will have communications (to receive a sleep=0), but reduced functions.
Therefore, updates should "wake the Nextion" (sleep=0) before update
then re-enter sleep mode after such updates are made.