Everyone is different, 7-7.5 is normal from what I remember, I like 8-8.5 One of my gifted friends in BB likes 9, I also know people who do well on 5.
If I don't get ~8 eventually my brain doesn't work as well, and my lifts aren't quite as good. I'm pretty sharp on 8 hours, after a few weeks on 6 except weekends I can still recite 90 decimal places of Pi but short term memory suffers.
If you interupt REM cycles (waking up) then you might feel like you missed an hour plus of sleep, missing 2 cycles for most is like missing 2-3 hours of sleep. This hits your short term memory and leaves you sleepy, especially if happening nightly. It takes me a good while to go back to sleep if I awaken, which is why I suffer so much, but some will hit the pillow and be out in 5 minutes. My serotonin/melatonin is just not where it should be.