If you factor in that during sleep your body is actively restoring your organs, and mind is flushing/filing the info collected during the wake hours. How many hours needed for that is an intriguing question. But hardly this time is wasted, unless your sleep is ... restless.

I recently watched an episode of "Nova" on the subject of sleep (Mysteries of Sleep), very convincing , even to some toddlers fussy about the sleep as usual :)