Sleep is a fairly complex activity when you understand the brain-body activity that occurs during various stages of sleep. For restorative sleep, all five stages are important. The five stages are:

• Waking: This is relaxed wakefulness and the body prepares for sleep. People fall asleep with tense muscles, and their eyes move erratically. As the individual moves closer to sleep, muscles relax and eye movement slows down.
  
• Stage 1: The first sequence is drowsiness. Polysomnography shows a 50% reduction in activity between wakefulness and Stage 1 sleep. The eyes are closed, but a person can be easily aroused. Stage 1 usually lasts 5-10 minutes.
  
• Stage 2: This period is one of light sleep. Polysomnographic readings show intermittent peaks and valleys. There are spontaneous periods of muscle tone mixed with periods of muscle relaxation. The heart rate slows and body temperature decreases. The body is preparing to enter deep sleep.
  
• Stages 3 and 4: These are deep sleep stages, with stage 4 being more intense. In both stages, the sleeper’s polysomnographic reading shows slow-wave or delta sleep.
  
• Non-Rem Sleep: This occurs in stages 1-4 and lasts from 90-120 minutes, each stage lasting between 5 and 15 minutes. A normal sleep cycle has this pattern: waking, stage 1, 2, 2, 4, 3, 2, REM sleep. REM sleep usually occurs 90 minutes after sleep has begun.
  
• REM Sleep: This period is one of rapid eye movements and intense dreaming. In most instances, muscle paralysis occurs in the major voluntary muscle groups. The first REM period of sleep lasts about 10 minutes, with the next REM period a little longer, until the final REM stage, lasting about one hour. An individual completes five sleep cycles during a typical night’s sleep.

Credit: www.sleepdisorderchannel.net