Answer: Sleep is a dynamic process. There are 2 distinct states that alternate in cycles and reflect differing levels of neuronal activity. Each state is characterized by a different type of brain wave (electrical activity that is recorded with the help of electrodes placed on the skull) activity. Sleep consists of non–rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM is further subdivided into the following 4 stages:Stage I (light sleep)Stage IIStage III & IV (deep sleep)The stages of NREM sleep and REM sleep cycle over and over again during a night's sleep. Stages I, II, III, and IV are followed by REM sleep. A complete sleep cycle, from the beginning of stage I to the end of REM sleep, usually takes about one and a half hours.For the purpose of analysis, a night’s sleep is divided into 3 equal time periods: sleep in the first third of the night, which comprises the highest percentage of NREM; sleep in the middle third of the night; and sleep in the last third of the night, the majority of which is REM. Awakening after a full night’s sleep is usually from REM sleep.