In Power-Down sleep mode, all peripherals are stopped, except for the Watchdog Timer (WDT) and the Periodic Interrupt Timer (PIT). The only wake-up sources are the pin change interrupt, the TWI address match, and the CCL with filter and edge-detect disabled.
In this mode, the High-Frequency Oscillator (OSCHF) is stopped, and only the 32.768 kHz oscillator is running.