I wonder why humans need to sleep at all considering our brains are just as active asleep as they are in waking hours.

Not the same parts are active, though.

Sleep gives the body a chance to repair itself and for the immune system to fight infection.

http://www.meaningofdreams.org/dream...m#whydowesleep

Dreaming is also essential, and it wouldn't be safe for us to walk around while we were dreaming; we need to be immobilized.

I find it interesting that predators sleep much more than herbivores - think of how much time a cat spends sleeping.

Of course, this is partly because herbivores need to stay awake and alert as much as possible, to avoid becoming prey, but I also think that hunting for food requires a higher level of mental ability, and sufficient sleep is needed to bring it up to the necessary level.