A sleep cycle is split into four phases. The first three phases are non-rapid eye movement, or NREM sleep. The first phase of the sleep cycle is light sleep while the second phase starts seeing an increase of neural activity. The third phase of NREM sleep is known as deep sleep and is crucial to rest properly and allow the body to heal. The final phase is known as rapid eye movement, or REM sleep. During this phase, the brain processes memories and new information.