I used an app on my iphone (4) that monitored how "deep" asleep I was, I don't know how reliable this is but there are many positive reviews of the app (sleep cycle).

I think the optimum time for me to wake up and go back to bed is at 4am? When Im most asleep? or is it the other way round, when Im most awake.

Anyway, try to interpret this graph (Went to sleep 12.37, woke up at 7.44, 7h)