How can you determine the exact amount of sleep you need a night?

I know 8 hours is the average, but it is different for everyone. When I think I have enough sleep, I end up dozing off during the middle of the day. When I sleep for a vey long time, I feel sluggish. Also, I read somewhere that if you are woken up while you are in R.E.M. sleep (deep sleep), you’ll feel groggy the whole day. Basically, I’m wondering how you can predetermine the ideal amount of sleep you need a night.