the other category, where this movement of the eyes does not happen, is called nrem-sleep (non-rem sleep). in general, dreams do not occur during this time. there are three or four stages of nrem-sleep. stage i is just barely sleeping, or dozing. stage ii is also light sleep. normally, in adult humans, about half of the time spent asleep is spent in light sleep. stages iii and iv are called deep sleep. deep sleep is necessary for growth and healing. it can be quite difficult to awaken someone who is in stage iii or stage iv sleep. sometimes, stages iii and iv are combined and called stage iii.