FIVE SLEEP STAGES

WAKEFULNESS:
The eye movement lines look like mirror images of hills and mountains. The brainwaves on the screen are small, rapidly but irregularly oscillating lines, like a very nervous person might make if trying to draw a straight line. This low, fast, random pattern of brain waves is called beta waves, and they indicate wakefulness.
The muscle tension recording does not look like a line at all, but rather a thick band made up of many vertical lines of random heights, somewhat like a magnified side view of a shag carpet.
Suddenly, all the lines become very wide, irregular, thick tracings. This pattern stops after a second or two as abruptly as it started, and the lines return to what they looked like previously: ‘‘What was that?’’ you blurt out. ‘‘She moved. She probably was trying to get more comfortable. We’ll occasionally see that all through the night. It actually is useful, because it tells us how restless the sleep is.’’
FALLING ASLEEP:
The EEG lines begin to change from their low, fast, random pattern of beta waves to higher, slightly slower, but very rhythmic and regular patterns, looking somewhat like a folded ribbon candy viewed from the side. ‘‘These patterns are alpha waves. They occur when a person’s mind is awake but relaxed and not particularly concentrating on anything.’’
When he flips the intercom switch and tells Joan to open her eyes, the alpha waves change back to the beta waves. ‘‘Alpha waves also occur when a person is drifting into sleep. What you saw before the alpha waves, and are seeing now, are the beta waves of an awake, alert mind.’’
ASLEEP:
In several minutes, the EMG becomes less thick to about half its original size, more alpha waves appear, and fewer eye movements can be noted. Then, the eye channels trace out lines that look like mirror images of rolling hills, and the EEG becomes much more jagged, but the waves are not as rapid as beta nor as rhythmic as alpha. ‘‘These waves are the signs of the start of sleep—so called slow rolling eye movements and the replacement of alpha waves with slower, less regular theta waves in the EEG. It’s a light sleep called N1 short for NREM 1; it was formerly called stage 1. She will probably spend very little sleep time in this stage. It’s more of a transition between stages.’’
Soon Joan’s eyes stop moving, and the EEG line gradually oscillates less rapidly than before, but it is still rather jagged. Then, it more rapidly and regularly oscillates for about a second, producing a wave that looks like compact alpha. ‘‘That’s a sleep spindle, a sure sign of N2 sleep formerly called stage 2 sleep.’’  Soon there is a sudden, large, upward movement, then down past midline, then back to its previous activity level, resulting in a pattern resembling an upside-down pointed ice cream cone next to a smaller but right side up cone. ‘‘That’s a K-complex, another characteristic of N2.’’
This pattern continues for another 10 min—occasional spindles and K-complexes on a background of irregular but slower and slower activity.
Then, the EEG begins to show occasional large sonorous movements and fewer spindles and K-complexes. When about 20 % of the record is of this pattern, Sam explains, ‘‘These are slow waves, also called delta waves. They indicate the presence of N3 sleep previously called slow wave sleep. In many ways, this sleep is the deepest sleep.’’
Soon much of the record contains delta waves and continues this way with little change for about half an hour. Suddenly, all of the lines become large and blurred indicating a body movement. We confirm this recording when we look up at the TV that is monitoring Joan and see that she is rolling over. When things settle down, the record again resembles N2 with moderate, jagged background and spindles and Ks.
DREAMING:
Exactly 93 min after sleep onset, the EMG becomes almost a thin, straight line. Suddenly, the eye movement channels burst into activity, showing large, jagged, mirror-image mountains for a few seconds, and then falling silent. ‘‘That recording is a burst of rapid eye movements. Joan is now in another kind of sleep called REMS for rapid eye movement sleep.
Look closely at the EEG. Notice no spindles or Ks are present and many of the brain waves look like the teeth of a saw blade and thus are called saw tooth waves.’’ Just then there is another burst of eye movement lasting longer than the first. ‘‘As you can see, sleep is not a single entity, but is made up of several different states.’’
The REMS period does not last long. After a few minutes, another body movement occurs and N2 returns for 10–15 mins followed by more N3 sleep. And so it goes throughout the night. Joan cycles between the stages, except there is less and less time spent in N3, in fact, almost none at all in the second half of the night, and more and more time in REMS. Most time, however, is spent in N2— about half of the night. Around every hour and a half, she starts a REMS period.
WAKING UP:
At 6:15 a.m., through blurry eyes, we can see that the pattern on the computer screen is changing. Several body movements occur, and the EMG gets thicker again. The EEG becomes low and fast and random, and Joan’s eyes start moving, but not as rapidly as during REMS and more continuously. ‘‘She’s awake now,’’Sam informs us. ‘‘Good morning,’’ he intercoms to Joan and turns on the light. ‘‘Ugh—oh, mornin’’’ (Yawn).

Excerpt from Moorcroft, W. H. (2013). Understanding sleep and dreaming (2nd ed.). Springer Science + Business Media. https://doi.org/10.1007/978-1-4614-6467-9

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