There are 4 stages when we sleep: 1,2,3 (Non-REM (NREM) sleep) and REM (rapid eye movement) sleep.
Stage 1 and 2 is called a light stage of NREM sleep. In stage 1, the transition between waking and sleep. If awakened, the person will claim he was never asleep. For stage 2, the main body of light sleep. Memory consolidation and synaptic pruning.
Stage 3 is known as deep NREM sleep. The most restorative stage of sleep, stage 3 consists of delta waves or slow waves. Awakenings or arousals are rare and often it is difficult to awaken someone in this stage
REM sleep, also known as rapid eye movement, is most commonly known as the dreaming stage. Awakenings and arousals can occur more easily in REM; being woken during a REM period can leave one feeling groggy or overly sleepy.
The sleep cycle is a progression through the various stages of NREM sleep to REM sleep before beginning the progression again with NREM sleep.
Typically, a person would begin a sleep cycle every 90-120 minutes resulting in four to five cycles per sleep time, or hours spent asleep.