For a theory of sleep to convince me, it would need to explain how the benefit from sleep can possibly be worth the huge disadvantage of leaving yourself comple

For a theory of sleep to convince me, it would need to explain how the benefit from sleep can possibly be worth the huge disadvantage of leaving yourself completely vulnerable for hours a day. Evolution-wise, that seems hard to justify. A variant of human that didn't have to sleep seems like it would overrun the sleep-needing humans, unless the non-sleeping human just isn't possible.