so, water-based animals sleep differently to land based animals. most whales and dolphins sleep with half their brain at a time so they can still be aware of their surrounding, and so they dont drown.

also, while orcas might attack a sperm whale calf, theyre not big enough to take down a fully grown adult, and its normally only the adults that sleep like this - calves tend to sleep while travelling next to their mothers. they may however have a 'babysitter' who looks out for the calves while the other adults sleep. hope that answers your question!