Rejuvenation occurs in stages;
sleep stages.  Skipping any of them can cause damage.

Before the discovery of sleep stages, scientists thought that sleep was just a continuous state of unconsciousness from dusk to dawn.  

The discovery that sleep progressed in stages was one of those accidental occurrences.  A doctor observed that at certain times, the eyes of sleeping babies would exhibit movement; sometimes a slow, rolling movement; at other times, rapid bursts of movement.

The doctor (Dr. Aserinsky) and his colleague, Dr. Nathaniel Kleitman, the father of sleep research, decided to explore this phenomenon in adults using the electroencephalograph (EEG).

By use of the EEG, changes in brain activity can be measured by placing electrodes at strategic points on the skull and recording the electrical activity, "brain waves", on charts.

The big discovery was that sleep goes through four stages then enters what is called REM sleep, or rapid eye movement.  Each stage has distinctive patterns that can be viewed and analyzed on the EEG.

In our normal waking state, the EEG graph is a low voltage, desynchronized electrical activity.

Photo: Five cycles of sleep over an eight hour period

Sleep follows a regular pattern of four stages followed by REM sleep

Slow-Wave Sleep Stages

The first stages are numbered one through four and called the slow-wave stages.  They are also known as non-REM sleep to differentiate them from the rapid-eye-movement stage.

Photo:  EEG Stage 1 sleep stage chart

EEG stage 1 sleep chart

Stage 1 starts in the 1st hour of sleep and shows a progressive slowing of EEG activity. Sleep is light and noises and various disturbances can awaken the sleeper.  Muscle activity starts to slow and eyes move slowly.

In sleep stage 2, eye movements stop and brain waves exhibit an
even slower pattern with only occasional bursts of rapid waves.

In stage 3, brain waves continue to slow but still are characterized by moments of smaller faster waves.

EEG Stage 4 sleep stage chart

By stage 4, the characteristic waking state wave has progressed from low voltage to high voltage and desynchronized to highly synchronized.  The physical manifestations of stage 4 sleep is a decrease in heart rate, blood pressure and respiration; very typical parasympathetic nervous system (PNS) response.

Photo:  EEG Stage 4 sleep stage chart

The PNS is the rest and recreation system.  Stages three and four are considered deep sleep and it is difficult to rouse someone from either of these stages. 

Sleep walkers and children that wet the bed, do so in these two
At end of stage 4, about 90 min into sleep, we go into the rapid eye movement (REM) stage.

REM Sleep Stages

REM is a deeper level of sleep but the REM EEG is very similar to awaking EEG.  It is very hard to awaken someone from REM sleep.  Physical characteristics are increased oxygen consumption, increased heart rate, Blood pressure and respiration.

Photo:  REM sleep waves outlined in red; eye movement above solid red line 

EEG REM sleep chart

REM sleep is sympathetic nervous system dominance; the fight or flight response.  Consumption of oxygen in REM sleep is actually higher than in physical exercise. 

It's amazing how the body works to protect itself. 

While wild dreaming is going on during REM sleep, our arm and leg muscles are temporarily paralyzed so we can't act out the dreams and possible hurt ourselves.

Because REM sleep is similar to the waking state, it is called paradoxical sleep and is the deepest level of sleep you can be in. 

During the course of the night there are five to seven complete cycles, each progressing through the four stages and REM.  Also during the progression of sleep cycles, the REM period gets longer and the early slow-wave stages get shorter.

It is believed that REM sleep is the time when the brain sorts and stores new information acquired during waking hours.  New pathways are being formed to enable learning and retention.

The MNT Knowledge Center has a comprehensive article on REM sleep accompanied by a short video. Pretty interesting things going on in our brains during that deep sleep REM stage.

Let's Take a Nap

After plowing through a discussion about sleep stages, one may be excused for feeling a need to take a nap.   It turns out that naps are almost as complicated as the sleep stages so before leaving this topic, let's consider napping.

There is definite benefits and even some downsides to napping, depending on how long one dozes off and what the nap is supposed to accomplish.   For simplicity it makes sense to tie the nap duration to the sleep cycles described above.

Photo: The Bumstead maneuver, 23 April 2009/Hostile Witness.  Courtesy Paul Epps;

When it comes to napping, no one does it better than Mr. Dagwood Bumstead. 

Most everyone is familiar with the term "power nap".  This is the short 10 to 20 minute nap that is best for a quick boost in alertness and energy.  It sure beats taking one of those high caffeine, high-sugar energy drinks.  Considering sleep stages, the power nap limits the snoozer to stage 1 or 2 of the first sleep cycle which means a light sleep with no rapid-eye movement.  The value is that one can quickly get back to work or whatever other activity is contemplated without any recovery period.

The 30 minute nap is generally counter productive in that it produces a feeling of being in a fog, sometimes referred to a sleep inertia, that can last for 30 minutes or so after waking up.  After shaking off the grogginess the benefits of the nap will become felt.

Then there is the 60 minute nap which probably would not be tolerated if done on the bosses time.   Since the 60 minutes will take one to the deepest slow-wave sleep, it's benefits include an enhanced ability to remember things like names, places, dates, faces and factual data.  There is a downside similar to the 30-minute nap, namely a period of grogginess after waking up.

Then there is the 90-minute nap which is ideal for that lazy Sunday afternoon.   It is long enough that it covers a sleep cycle including the rapid-eye movement phase usually associated with dreaming.  It is easier to wake up without the sleep inertia grogginess and enhances ones creativity as well as emotional and procedural memory.

Sleep researchers have noted some interesting things about napping, such as:

  • Younger people require longer naps and as we age, we seem to get by just fine with the shorter naps
  • It also seems that people who have engaged in exercise or a vigorous workout in the past 24 hours or so tended to nap more than those who did not exercise
  • Napping between 1:00 pm and 4:00 pm seems to be the ideal time for a nap
  • A little over one-third of U.S. adults claim to nap on a typical day
  • People who tend to dream during short naps are usually sleep deprived
  • Napping is not recommended for people experiencing sleep disorders such as insomnia or sleep apnea

  • You can keep on surfing the site, but I feel a nap coming on.

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