Trouble with sleep mode and interrupts
Only low level detect is possible with external interrupt pins (INT0/D2 and INT1/D3) in power down sleep mode. The signal must be low for a number of clock cycles to be detected and the interrupt will detected while the signal is low (and the interrupt is enabled). It is not a transition. 

A pin change interrupt could be used instead. 

For more details please see chap. 9.5 Power-down Mode and chap. 12. External Interrupts. 

Cheers!