optimal sleeping time

Do you believe that there's actually an optimal sleeping duration for each individual? You know how some people claim that we should sleep in multiples of a certain number, and that number varies from person to person?

I'm starting to think that mine's in multiples of 5! haha=) A few days back I had such a lengthy phone conversation with Vina, we only stopped talking at almost 4am... In the end I barely got 5 hours of sleep and went to school and work, etc. and it all worked out well. Yesterday, I attempted to sleep early...1am...and set my alarm for 6 or 7 am, I can't remember. And amazingly, I woke up effortlessly. But being the lazy and greedy for sleep me, I decided to give myself more sleep time. So I switched off the alarm and slept longer. And that's when everything starts to go wrong. I woke up just before class... but feeling much more tired now than yesterday.

Maybe I should tell my brain to stop taking pity of my body and just make do with 5hrs... Afterall, it's good training for finals=)