figure 9.6 Sleep stages Recordings of brain activity obtained while people sleep reveal the existence of distinct sleep stages. There are three stages of non-REM (i.e., no rapid eye movement) sleep: NREM-1, NREM-2, and NREM-3. Each one, in order, is a progressively deeper stage of sleep. After NREM-3, people move into a stage of REM sleep. During REM sleep, the brain is highly active, much as it is when people are awake.