Thought of the Day - Required Sleep

How much sleep is enough sleep and what determines how much you need? How do you optimise your sleep and more importantly (in a six-sigma-geeky kinda way) how do you measure how well rested you are?

On Monday got up at 5am after 4 hours sleep and felt fine all day till 10pm. Today got up at 7am after 8.5 hours sleep and been feeling a bit tired, not a lot but a bit. Twice as much sleep, two extra hours in bed in the morning but i don't feel much better.

Now i have improved my diet and i definitely need less sleep and feel less tired but is it optimised enough? I know i should exercise and a lot of people suggest that you will sleep better and feel more rested after that but then a lot of people also say that you need more sleep when you exercise to let your body recover. Again it is something i need to do in the long run (probably once the skin improves) but is it worth doing now to get more available time, energy and motivation for other things ...