One easy way would be to increment a counter variable in your Timer1 int handler, and turn off all LEDs & enter sleep mode once it reaches a certain value.

During normal operation, reset the counter var in the INT_INT handler.

Timer1 increments counter on each interrupt, external interrupt clears it. Just monitor the count value in your main loop.

Just remember that if you enter sleep mode, Timer1 stops unless you have an external crystal for your Timer1 clock source.