But apart from these findings, what else do we know about human sleep with any kind of certainty? It is known that humans sleep, like other mammals, according to a daily cycle. Once asleep, they switch between four different stages of unconsciousness, from stage one sleep, the shallowest, to the stage four, the deepest. When dreams occur, which is usually during the lightest sleep, the brain paralyses the body except for the hands and eyelids, thus preventing injuries.