According to the National Sleep Foundation, normal sleep consists of two major states: Rapid Eye Movement (REM) sleep and nonREM (NREM) sleep. NREM sleep accounts for approximately 75 percent of total sleep and is divided into four sleep stages, numbered stages 1, 2, 3 and 4.

The NREM state begins with stage 1 and deepens with each progressing stage – with brain waves becoming progressively larger and slower.

• Stage 1 consists of drowsiness
• Stage 2 consists of light sleep and mild muscle relaxation. The heart rate slows and the body temperature decreases as your body prepares to enter deep sleep.
• Stages 3 and 4 provide the deepest sleep (called delta sleep).

According to the Society for Neuroscience, the pattern reverses after reaching stage 4 and sleep becomes progressively lighter until REM sleep occurs. For example, a typical sleep cycle has this pattern: stages 1, 2, 3, 4, 3, 2, and then REM sleep.

REM sleep usually occurs about 90 minutes into the sleep cycle. It is the most active period of sleep and is characterized by intense brain activity and rapid bursts of eye movements. Also in this state, muscles may twitch, heart rate and respiration increase, and dreaming occurs. The first period of REM usually lasts about ten minutes, and each subsequent REM stage becomes longer.

The five stages of sleep – including the repetition – occur cyclically. The first cycle, which ends after the completion of the first REM stage, usually lasts for 100 minutes. Each subsequent cycle lasts longer as its respective REM stage extends. The normal person may complete approximately five cycles during a typical night’s sleep. 

Sources: National Sleep Foundation, Society for Neuroscience, Sleep Channel