I wanted to join in echoing Anders comments. In the past I have
had the ability to stay up for extended periods. It requires
however an intense focus to the exclusion of everything else.
Now-a-days, I'm partial to his biphasic sleep approach. I
did read a recent article about the fact that anthropologists
are now starting to study sleep in "primitive" cultures and
are finding that the traditional Western 8-hours a night
*is not* a "normal" sleep pattern. More normal is to sleep
whenever you feel the need to do so, wherever you feel like it, taking into account the need for safety, perhaps sleeping in "sub-groups" (some sleep, while others watch).
I concur with his comments on the primary purpose of sleep being to "integrate" critical experiences into your knowledge database. There may also be a biological purpose for maintenance, repair & recycling to be done when resources (esp. energy) are less needed for "thinking" (though I can't point to much hard biological evidence for this).
There is a piece of urban folklore that I put together from a number of sources -- Eastern mystics, gurus, etc. apparently need very little sleep (2-3 hours a night), while sociopaths confined to mental institutions sleep much more (12+ hours a day). Now, one would argue that the difference between these indviduals is their "closeness" to reality. The mystic really "sees" reality and accepts it completely. The sociopath has an internal "reality" that is much different from that which actually exists externally. If this is accurate, the sociopath needs much more sleep (processing time) to attempt to bend/integrate actual reality to his internal view of it while the mystic needs very little.
So if you want to sleep less, you have to minimize the differences between the external reality and internal reality. Whether you choose to modify the external reality (minimizing perceived threats, making it more like your "ideal", etc.) or your internal reality (stop worrying about things you can't change, "delete" unsupportive thoughts or behaviors, etc.) probably depends a lot on your individual situation.
However, once you have minimized this differences and life is very "comfortable", I think some people would want to sleep more. The energy costs of controling the internal reality completely (in sleep) are much lower than those required for controlling the external reality (where you have to push matter around), so you can experience more if you do it entirely internally.
I can state personally, that a couple of nights ago I got woken up from a dream that I *really* *really* would have like to have had continue just a bit longer. Reality bites.
Anders may be right, SIs do nothing but sleep.