sentence: Deep sleep is necessary for growth and healing.
sentences_before: 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.
sentences_after: It can be quite difficult to awaken someone who is in stage III or stage IV sleep.
concept_name: deep sleep
headings: Sleep Different categories of sleep
categories: Basic English 850 words VOA Special English words