Ok basically what I mean is when they say you need 8 hours sleep, people mean falling asleep at say 11:00pm and waking up at 7:00am right?

Now I know when you first fall asleep there's REM sleep for a while and stuff before you get into proper deep sleep. I assume it's the deep sleep your body needs the most, just not 8 hours of it?

What I'm actually asking is say REM takes two hours(no idea how long it takes) and I'm actually in deep sleep after that. If I'm woken up by say a noise or my cat wanting in my room but then go straight back to bed after say 30 seconds and fall to sleep again do i have to go right through that two hour REM period or does your body fall back into deep sleep much quicker because you were not conscience for long?