According to the BBC, albatrosses can sleep while flying. This is an amazing thing - though I imagine that once explained, it seems normal and well-within the bounds of possibility.
To say "albatrosses can fly while sleeping" would sound much less credible, as it conjures up images of sleepy birds taking off, flying, and landing in their sleep. Sleep-flying - like some of us sleep-walk. No, they probably soar up 1000's of meters in the air, find something like the Jet Stream, and put their wings in idle. I wonder if, while they are asleep and flying, do they have dreams of flying? From watching cats and dogs sleep, I am utterly convinced that they dream about running, barking, growling and meowing, so I imagine albatrosses dream of flying, too.