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 invention of EEG
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.
4 Stages of 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.
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.
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.
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 stages.
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.
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.