Puts the MCU in deep sleep mode. The deep sleep mode allows power optimization with the slowest wake-up time. All but the RTC peripherals are stopped. The CPU can be wakeup only using RTC or wakeup on interrupt capable pins.
LowPower.deepSleep();
LowPower.deepSleep(milliseconds);
milliseconds: the number of milliseconds to put the board in deep sleep mode. If void the deep sleep mode is used till a wakeup event.