OKAY so I'm doing this project on an attiny chip programing it with the Arduino as isp it runs 24/7
the programs like this :
digitalWrite(0;high)
delay(3hours;;millesecond equivalent)
digitalWrite(0;low)
I need to put attiny into sleep mode for 18 hours here then wake up and redo the loop
or basically attiny: turn on led for 3 hours_ turn off_ led sleep attiny for 18 hours_wake it up_redo
google is your fried. Search for 'attiny sleep'
Here's one example: H2OhNo! - SparkFun Learn
If all you want to do is re-run the loop, the watchdog timer can reset the system. To get to 18 hours, you will have to have a counter to get to that amount of time.
Check out the Narcoleptic library. You just replace delay() with Narcoleptic.delay() and then it just works! I have an IChing box that has been running for a year and a half on the same two aaa lithium Energizer batteries that uses Narcoleptic.delay().
I first read about Narcoleptic in Simon Monk's book Arduino Next Steps.