Normally, it is better to discuss real research.
This page has a lot of PDF's on the topic of sleep deprivation and performance:
http://www.spokane.wsu.edu/researchoutreach/Sleep/research.asp
This one:
Belenky G, Wesensten NJ, Thorne DR, Thomas ML, Sing HC, Redmond DP, Russo MB, Balkin TJ (2003). Patterns of performance degradation and restoration during sleep restriction and subsequent recovery: a sleep dose-response study. Journal of Sleep Research 12: 1-12
Finds that people restricted to regimes of 3,5,7 hours of sleep per night all showed varyling levels of decreased performance on tests designed to measure motor skills and attention. Less sleep == more degradation. Another group, where sleep was restricted to 9 hours showed no degradation at all compared to baseline.
I don't know about exactly 8 hours of sleep, but there probably is a point somewhere between seven and nine hours of sleep that is the break-even point - any less shows performance degradation, any more keeps baseline performance.
So I would say '8 hours of sleep is probably a good starting point for assessing your need for sleep'.