We spend about one-third of our lives sleeping. We know relatively little about sleep.

A We know relatively little about sleep; as a result, we spend about one-third of our lives sleeping.

B  We shall know more about sleep if we spend more than one-third of our lives sleeping.

C Despite spending about one-third of our lives sleeping, we know relatively little about sleep.

D We spend about one-third of our lives sleeping so that we know relatively little about sleep.