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

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

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

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

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