From a survey of all the 'A' level top scoring friends that I have, I have noticed a common advice they always issue: sleep at least 7hrs.

Here's a sample of how these guys have fared in their 'A' levels in 2005:
Anglo Chinese Junior College, 9 Distinctions, awarded the prestigious Medical PSC scholarship
Hwa Chong Institute, 9 Distinctions, A*Star scholarship
Victoria Junior College, 7 Distinctions, 2nd-tier Police Overseas Scholarship

These people sleep at least 7hrs a day before their lessons the next day. This is so that they could concentrate thoroughly in class and acquire full understanding of the topic at the lesson. The full, uninterrupted rest obviously gave them the ability to focus so intensely in class.

However much I mugged in JC, I could produce only 4 distinctions in my A levels. I noticed the very grave mistake I made in staying up till 2 or 3am everyday and dozing off in classes. Not a good strategy. If only I had the chance to learn from these successful scholars earlier.