Read section 10.2 of the datasheet!
"If BOD is disabled in software, the BOD function is turned off immediately after entering the sleep mode. Upon wake-up from sleep, BOD is automatically enabled again. This ensures safe operation in case the VCC level has dropped during the sleep period.
When the BOD has been disabled, the wake-up time from sleep mode will be approximately 60 μs to ensure that the BOD is working correctly before the MCU continues executing code.