How to turn on ATmega every 6 hours?
You can use the Watchdog timer to issue an interrupt instead of reseting the chip.
The chip will wake up even from the deepest sleep mode on the watchdog interrupt. 

You will want to set the biggest prescaler for the Watchdog timer (8 seconds) and count interrupts. If the counter has not yet reached the predefined amount, immediately go back to sleep, otherwise, execute logging code.