How to configure EXIT from Sleep Mode on UART Activity?

[S.M.]

Hi,

I am using B-L072Z-LRWAN1and AT-Slave project from I-Cube-LORAWAN SDK. I can see the board keeps entering sleep mode and its been set for WFI but not sure which interrupt cause it to exit from sleep mode. How can I check which interrupt cause it to exist from sleep mode and how can I configure the sleep mode to exit only when there is data on UART line??