But sleeping makes you much more vulnerable to predation, and really doesn't save much more energy vs. quiet/restful wakefulness. Just think of how disoriented

But sleeping makes you much more vulnerable to predation, and really doesn't save much more energy vs. quiet/restful wakefulness. Just think of how disoriented you are when something wakes you up in the middle of night - that's a definite biological cost if that thing is something threatening to you. And of course, never mind creatures that have gone as far as the 50/50 brain adaptation (IE Dolphins), they simply have to be at least partially awake all the time.