Ever had a sleepless night before an exam, cramming in the things you didn't learn in time? Even after 40 hours without sleep, it is still possible to disgorge crammed information. But remember those facts a week later? Forget it.
Ever spent so long playing a video game that residual images of the game permeate your dreams?
Ever heard your cat mew in its sleep and wondered if it is having a troubled sleep? Are animals intelligent? Do sheep, for example, dream? Probably not much: Sheep have only half an hour of REM sleep per night.