Right. He said some people experience great difficulty in trying to float, whereas others manage it very easily – something to do with the fat and muscle ratio of individual bodies affecting buoyancy. The time factor involved for survival would obviously depend upon the possibility of succumbing to hypothermia and the ability to survive without drinking water. If prevailing weather conditions/currents resulted in very calm water, it would be possible for someone who is able to relax and to float well, to sleep. A choppy sea would mean water sploshing over the face at regular intervals so wouldn’t be conducive to a peaceful snooze. He put it all rather more technically, but I’m sure you get my … err … drift.