The conceptual application does not need a BOD in sleep. Disabling the BOD in sleep will reduce the power consumption.
In active mode, the conceptual application can use the Enabled with wake-up halted mode. This configuration means that the BOD will monitor the supply current when needed (active mode), but will not be active in sleep mode. Supply current is checked when a wake-up occurs, so that normal operation is safe. There will be a small penalty of added wake-up time, but as the example application is not time critical, this is tolerated.