The sleep/wakeup functionality of an Atmega328p is normally used for extreme power saving in battery powered systems. The Uno is not appropriate for this because it has lots of power hungry components like voltage regulator, sub chip, LEDs etc. The few milliamps saved by using power saving tricks doesn't make it worth while.
To wake the chip from sleep using an interrupt, it has to be an external interrupt (pin 2 or 3) eg
attachInterrupt (0, wakeMe, LOW); // pin 2
Why are you doing this? If it is not for power saving, there may be a better way.