What are the five stages of sleep?

As a subject matter expert in sleep science, I can provide you with an overview of the five stages of sleep as understood through current research. Sleep is a complex physiological process that is typically divided into two main categories: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is further divided into three stages, while REM sleep is the fifth and final stage. Here's the breakdown: 1. Stage 1 NREM Sleep: This is the lightest stage of sleep, during which the brain transitions from wakefulness to sleep. Muscle activity, eye movements, and heart rate begin to slow down, and the individual may experience a sensation of relaxation or drowsiness. 2. Stage 2 NREM Sleep: Deeper than the first stage, this stage is characterized by a further slowing of brain waves, with occasional bursts of rapid brain activity known as sleep spindles. It is considered a period of light sleep where the body's temperature begins to drop. 3. **Stage 3 NREM Sleep (also known as Delta or Slow-Wave Sleep)**: This is the deepest stage of NREM sleep, where delta waves dominate the brain's activity. It is the most restorative stage of sleep, associated with physical healing and growth. 4. Stage 4 NREM Sleep (REM Sleep): This stage is a bit of a misnomer as it is not a separate stage but rather the transition into REM sleep. During this stage, the brain's activity increases and becomes similar to the wakeful state, while the body's muscles become very relaxed, almost paralyzed, which is known as REM atonia. 5. REM Sleep: The fifth and final stage is when most dreaming occurs. The eyes move rapidly under the closed eyelids, while brain activity is high and respiration and heart rate can become irregular. REM sleep is crucial for learning, memory consolidation, and emotional regulation.

Usually sleepers pass through five stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. These stages progress cyclically from 1 through REM then begin again with stage 1. A complete sleep cycle takes an average of 90 to 110 minutes, with each stage lasting between 5 to 15 minutes.