Before revision: Necessity If sleep were not essential, one would expect to find Animal species that do not sleep at all Animals that do not need recovery sleep after staying awake longer than usual Animals that suffer no serious consequences as a result of lack of sleep Hence sleep is essential for all complex animals .
After revision: Necessity If sleep were not essential, one would expect to find Animal species that do not sleep at all Animals that do not need recovery sleep after staying awake longer than usual Animals that suffer no serious consequences as a result of lack of sleep These symptoms are not seen in complex animals, and is thus considered necessary .