The Cleveland Clinic

Sleep Disorders: Sleep 101

Introduction to sleep

When you sleep, your body rests and restores its energy levels. However, sleep is an active state that affects both your physical and mental well-being. A good night's sleep is often the best way to help you cope with stress, solve problems, or recover from illness.

What Happens During Sleep?

Sleep is prompted by natural cycles of activity in the brain and consists of two basic states: Rapid Eye Movement (REM) sleep and Non-REM (NREM) sleep, which consists of Stages 1 through 4.

During sleep, the body cycles between non-REM and REM sleep. Typically, people begin the sleep cycle with a period of non-REM sleep followed by a very short period of REM sleep. Dreams can occur in any stage of sleep.

What Is Non-REM Sleep?

The period of NREM sleep is made up of stages 1-4. Each stage can last from 5 to 15 minutes. Stages 2 and 3 repeat backwards before REM sleep is attained.

  • Stage 1: Polysomnography (sleep readings) shows a 50% reduction in activity between wakefulness and stage 1 sleep. The eyes are closed during Stage 1 sleep. However, if aroused from this stage of sleep, a person may feel as if he or she has not slept. Stage 1 may last for five to 10 minutes.
  • Stage 2: This is a period of light sleep during which polysomnographic readings show intermittent peaks and valleys, or positive and negative waves. These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and the body temperature decreases. At this point, the body prepares to enter deep sleep.
  • Stages 3 and 4: These are deep sleep stages, with stage 4 being more intense than Stage 3. These stages are known as slow-wave, or delta, sleep. During NREM sleep, the body repairs and regenerates tissues, builds bone and muscle, and appears to strengthen the immune system. As you get older, you get less NREM sleep. People under age 30 have about two hours of restorative sleep every night, while those over 65 may get only 30 minutes.