It might give you insight to look up Circadian rhythms. Much of the sleep cycle is regulated by light and dark cycles. I recall reading an article where they did an experiment with humans with no changes in light cycle and no access to clocks and they settled into a 18 hour sleep wake pattern.
This might make what bowlturner said be the way to go. Think about how creatures would evolve with that light/dark cycle you mention above. Also note in the wikipedia article they mention that some reindeer above the arctic circle only experience circadian rhythms when the dark/light cycle is around, and none when they are in long seasons of dark. they would still need sleep but no longer are affected by the cycle.
Also look at some of the benefits of sleep. In some teaching and training circles it is suggested it allows for diffuse learning to take place (thoughts assimilated in patterns etc) and also for toxins to be flushed from the mind. Is there some other way that this can be accomplished through advanced technology? In which case sleep might just be something people indulge in for sensual pleasure (controlling their own dreams etc)