Understanding cycles of sleep

Sleep is far more than just an unconscious mental state. Sleep researchers have determined that several stages of sleep with varying amounts of physical and mental activity occur predictably each night, and these stages are incorporated into sleep cycles that last roughly 90 minutes each.  These stages of sleep have been extensively studied, and are divided into two main types –

Rapid Eye Movement (REM) sleep, and

Non-Rapid Eye Movement (NREM) sleep.

REM sleep is characterized by tiny back-and-forth movements in the eye, and typically are accompanied by the visual and emotional world of vivid dreams. The eyes move in tandem, as they do normally during an awakened state. REM sleep typically comprises about 20-25% of sleep for most adults, although that percentage declines somewhat as we age. All of us dream, even though we may not remember the dreams.

REM sleep includes very active brain activity, and if awakened during REM sleep many people will vividly remember their dreams. Our bodies are also much more active during REM sleep, with a higher heart rate, breathing, and blood pressure than during deep sleep.

REM sleep is also characterized by a natural form of muscle paralysis.  A deep inner part of the brain known as the pons blocks signals from the brain to the spinal cord, resulting in a form of paralysis. Sleep scientists theorize that such muscle paralysis is necessary to keep us from physically acting out our dreams.

NREM sleep is generally associated less with vivid dreaming, although studies have shown some form of dreaming activity to be a continuous process during sleep.  NREM sleep is often divided into 4 distinct stages that represent different states of body and brain activity while falling asleep and during deep sleep.

Stage 1 sleep is drowsiness. It typically lasts about 5 to 10 minutes. The eyes are closed, but if awakened a person probably would say he or she has not slept. One of the goals of the STS is to help you more easily drift into this first drowsy phase of sleep, which then sets the stage for true sleep.

Stage 2 is deeper than Stage 1 drowsiness and a form of true sleep. The heart rate slows, respiration becomes slower and deeper, and body temperature decreases as it gets ready for deep sleep. However, an external noise or disturbance would easily awaken us from this light form of sleep. Stage 2 sleep may last 30 to 45 minutes.

If at times you feel you are lying in bed but not sleeping, yet somehow the hours seem to go by quickly – you probably are in fact lightly sleeping in Stage 2 or deeper. Keep this in mind as you work the STS Stage 2 sleep counts as legitimate sleep. It is serving its purpose of helping refresh and rejuvenate you for another day.

Stages 3 and 4 are deep sleep, sometimes called slow-wave sleep, characterized by deep slow breathing and regular continuous rhythms of brain activity. Stage 4 shows deeper patterns than stage 3. We are for the most part cut off from the stimulation of the external world in deep sleep, and least likely to be awakened by a noise or other disturbance at these stages. The deep sleep stages may last around 45 minutes or so.

After stage 4 is reached, stages 3 and 2 repeat, then a period of REM or dreaming sleep is attained. So a typical progression is 1-2-3-4-3-2–REM.

In a typical night, each of these sleep cycles lasts from 90 to 110 minutes on average. The first few cycles of the night typically include longer stages 3 and 4, meaning the deepest sleep, with less dreaming. Later in the night, the amount of REM sleep and dreaming increase in each successive cycle. Typically a person will have four to six of these cycles each night, and the amount of REM dreaming on the final cycle will be around an hour.


                                                            Source:  National Institutes of Health

A typical night for a young, healthy adult, showing 5 cycles of REM and NREM sleep. Light-gray areas represent non–rapid eye movement (NREM) sleep.

Clinical research suggests that deep sleep (Stages 3 and 4) is the most important stage, in that those that are sleep deprived first make up time spent in deep sleep. During deep sleep, blood flows primarily to the muscles and not the brain. It is during deep sleep that our physical bodies are replenished with energy, and our immune systems are most capable of fighting illness.

The function of REM sleep is less understood, although research suggests both REM and NREM sleep help consolidate memory and facilitate learning. For most people, REM may also help to in effect reset the emotional state, in that lack of REM may result in one feeling more irritable or grouchy the next day.

In a simple way then, we might look at NREM or deep sleep resetting the physical body, and REM or dream sleep resetting the mind, and especially one’s mood, for a new day.

If you have been experiencing sleep problems, we invite you to check out the extensive scientific information included in the Sleep Training System (STS).  Taking the time to understand more about how your unique sleep system works will help you feel more relaxed and confident about working with and improving your own sleep system.

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16 Comments on “Understanding cycles of sleep”

  1. […] times each night.  Generally, normal sleepers briefly awaken at least five or six times between sleep cycles each night, and then fall back asleep within seconds.  Typically these very brief periods of […]

  2. […] common and normal to awaken between cycles of sleep.  We cycle through 5 or 6 of these complete stages each night, and sleep is lightest between […]

  3. […] sweats often occur between cycles of sleep.  That’s when sleep is typically lightest and we experience the least self-control.  Normal […]

  4. […] are is a manifestation of REM sleep, and REM sleep is good.  It’s healthy.  To reach a REM stage of sleep, we normally go through all the preceding stages of NREM sleep, also known as deep or slow wave […]

  5. […] like 10 to 15 times per night.  It’s common and normal to briefly awaken, especially between cycles of sleep. Most people usually forget about these transient awakenings and it’s not an issue.   Now […]

  6. […] (Rapid Eye Movement) sleep normally occurs after the deeper NREM (Non-Rapid Eye Movement) stages in predictable cycles each night.  That’s why you can take some satisfaction in knowing that if you’re […]

  7. […] for sleep you are describing, when your mind and body would presumably go through several deeper NREM-REM cycles.  That would understandably reduce sleep drive significantly that night, making it much harder to […]

  8. […] are a form of REM dreaming.  While dreaming is thought to be a continuous process throughout the night, our most vivid dreams […]

  9. […] better understand what’s going on, consider the normal sleep cycles we typically experience each night.  It’s normal to briefly awaken between the 5 or 6 cycles […]

  10. […] all dream, even if we don’t remember our dreams upon awakening.  Dreaming is an inherent part of the sleep experience.  We can’t not dream, just as we can’t not […]

  11. […] shifts within our brains, and various chemicals naturally ebb and flow throughout our bodies in regular cycles of distinct sleep stages each […]

  12. […] times each night.  Generally, normal sleepers briefly awaken at least five or six times between sleep cycles each night, and then fall back asleep within seconds.  Typically these very brief periods of […]

  13. […] description suggests you may be awakening consistently after your third or fourth sleep cycle, and probably right after a REM dream phase when sleep is lightest, and before a new roughly 90 […]

  14. […] sleep is also reduced by roughly half or more as we age, as is the amount of time we spend in the deepest NREM stages.  The elderly in fact may spend the majority of the night in stage two, the lightest form of true […]

  15. […] you are allowing enough time in bed for your last sleep cycle to complete, sleep inertia — that feeling of sleepiness you described — is reduced.  In […]

  16. […] time for proper sleep —  you will awaken spontaneously and refreshed after the last of your sleep cycles completes, with no alarm clock at […]

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