This is a bit random but its been bothering me for a while...

 

How do aquatic mammals (e.g. dolphins) who do not have gills, sleep if they have to constantly resurface for air?

from what i have read somewhere dolphins will automatically float back to the top and breathe in air while asleep.

would that not make them easy targets for predators?

They don't sleep in the same way we do. They have to be conscious to breathe, and so they can never be completely unconscious. They are still mammals, though, so their brains do need to be unconscious for several hours a day in order to continue functioning properly. They solve this by letting only half of their brains "sleep" at a time. Humans have no equivalent state, and so we can't describe what it feels like, but behavior-wise, they either swim very slowly near the surface, occasionally surfacing, or just float on the surface. Since they are also literally "half-awake," they can return to full consciousness the moment any trouble appears.

Interesting, thanks