When the SM2..0 bits are written to 010, the SLEEP instruct
ion makes the MCU enter power-down mode. In this mode, the external
oscillator is stopped, while the external interrupts, the 2-
wire serial Interface address watch, and the Watchdog continue operating
(if enabled). Only an external reset, a watchdog system reset, a
watchdog
interrupt, a brown-out reset, a 2-wire serial interface address match,
an external level interrupt on INT0 or INT1, or a pin change interrupt
can wake up the MCU.
Das ist doch eigentlich recht verständlich.