Chapter 53. Stages of Sleep

dream

sequence of mental images and thoughts that occurs periodically during sleep

electroencephalogram (EEG)

a method of measuring the electrical activity of the brain, as recorded from electrodes on the scalp

neuron

a single nerve cell, forming the basic unit of the nervous system

NREM-1 sleep

stage of light sleep without rapid eye movements, characterized by relatively small and irregular EEG waves

NREM-2 sleep

stage between light sleep and deep sleep, characterized by medium-size EEG waves without rapid eye movements, and occasional bursts of high activity called sleep spindles

NREM-3 sleep

stage of deep sleep without rapid eye movements, characterized by large, slow EEG waves

REM sleep

stage of sleep characterized by rapid eye movements and small, irregular EEG waves; most dreams occur during this stage

sleep cycle

a regular pattern of changes in brain activity that occurs 3 to 5 times in a normal night of sleep

Stages of Sleep

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The figure includes a woman sleeping. She has electrodes on her forehead, which are connected to an EEG machine. The readout from the EEG machine appears at the top of the page and is labeled as EEG wave tracing. The waves start as small frequent spikes in NREM-1. The waves become taller during NREM-2. Then they become wider and taller in NREM-3. The waves get smaller as they enter NREM-2 again. Then the waves become even smaller in REM.

Learning Objectives:

Identify the stages of the sleep cycle as indicated by EEG waves.

Describe the general relationship between the shape of the EEG waves and the depth of sleep.

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1. You will spend about one-third of your life asleep. Have you ever wondered what your brain is doing while you are sleeping?

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2. In the mid-1900s, researchers discovered that the brain gives off weak electrical signals generated by the simultaneous firing of millions of neurons. Electrodes placed on the scalp can detect these signals, which provide a graphic record—an electroencephalogram (EEG)—of the overall activity of the brain.

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The figure includes various lines of EEG waves. The small fast waves represent light sleep. The large slow waves represent deep sleep.

3. When sleep researchers examined the EEG brain waves across an entire night’s sleep, they found a clear pattern: Slower waves are associated with greater relaxation and deeper sleep.

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The figure includes various lines of EEG waves associated with being awake, NREM-1, NREM-2, and NREM-3. The small fast waves represent being awake or in light sleep in NREM-1. In NREM-2 the waves become larger and slower, with spurts of small fast waves labeled as “spindles”. The large slow waves represent deep sleep in NREM-3. A graph on the right side of the screen shows the time it takes to enter each sleep stage. Within 30 minutes we go from being awake to NREM-1 and then to NREM-2. We stay in NREM-2 for 30 minutes before entering NREM-3, where we stay for 15 minutes before entering NREM-2 again.

4. Brain activity gradually slows as a person drifts off to sleep, then gets steadily slower and more regular over the next half-hour or so, as the person moves through three stages—from light sleep to deep sleep—called NREM-1, NREM-2, and NREM-3. This pattern then reverses, and the person moves from deep sleep into lighter sleep.

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The figure includes various lines of EEG waves associated with being awake, NREM-1, NREM-2, and NREM-3. The small fast waves represent being awake or in light sleep in NREM-1. In NREM-2 the waves become larger and slower, with spurts of small fast waves labeled as spindles. the large slow waves represent deep sleep in nrem-3. a graph on right side of screen shows time it takes to enter each stage. within 30 minutes we go from being awake nrem-1 and then nrem-2. stay nrem-2 for before entering nrem-3, where 15 again. us staying about rem. during rem brain are very short fast. returning back nrem-2.

5. Instead of returning to NREM-1, the sleeper goes into a very different stage called REM sleep, accompanied by "jerky" eye movements, small, fast EEG waves, and vivid dreams. During REM sleep, the brain is highly active, but the body (except for the eyes) is almost completely paralyzed.

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The figure is a line graph of the progression through the full sleep cycle. The X axis lists the hours of sleep from 0 to 8. The Y axis lists the four major sleep stages from top to bottom as awake, NREM-1, NREM-2, NREM-3. Within 30 minutes of sleep we go from being awake to NREM-1 and then to NREM-2. We stay in NREM-2 for 30 minutes before entering NREM-3, where we stay for 60 minutes before entering NREM-2 again. We don't stay in NREM-2 but transition into REM. We stay in REM for 15 minutes before returning back to NREM-2 for 30 minutes. We then enter NREM-3 again 3 hours into sleep. We continue this same process, with the time spent in REM getting longer, until around 6 hours into sleep. At this time we no longer enter into NREM-3 but transition between NREM-2 and REM.

6. The full sequence (dipping into deeper sleep, and then lighter sleep followed by REM sleep) is called the sleep cycle, and is repeated 3 to 5 times in a typical night. Notice that most of the deep sleep (NREM-3) occurs in the first half of sleep, while more REM sleep occurs in the last few hours.

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Practice 1: Exploring the Sleep Stages

Select each EEG brainwave to view a brief description of the associated stage of sleep.

Description:

NREM-1: stage of light sleep without rapid eye movements, characterized by relatively small and irregular EEG waves

NREM-2: stage between light sleep and deep sleep, characterized by medium-size EEG waves without rapid eye movements, and occasional bursts of high activity called sleep spindles

NREM-3: stage of deep sleep without rapid eye movements, characterized by large, slow EEG waves

REM: stage of sleep characterized by rapid eye movements and small, irregular EEG waves; most dreams occur during this stage

The figure includes four lines of EEG waves associated with being in NREM-1, NREM-2, NREM-3, and REM. In NREM-1, the small fast waves represent being in light sleep. In NREM-2, the waves become larger and slower, with spurts of small fast waves labeled as 'sleep spindles'. In NREM-3, the large slow waves represent deep sleep. In REM, there are very small and fast waves.

NREM-1

NREM-2

NREM-3

REM

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Practice 2: Observing the Sleep Stages

Select the PLAY button to view the sequence of stages in the sleep cycle across an entire night.

In a typical night, you pass through each of these stages of sleep several times in a regular pattern called the sleep cycle.

During which half of the night do you experience the most slow-wave, deep sleep (NREM-3)? During which half of the night do you experience the most REM sleep?

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Quiz 1

Drag each sleep stage label to its EEG wave. When all the labels have been placed, select the CHECK ANSWER button.

Select the NEXT button and move to the Quiz 2.

Perhaps you should go back to review EEG waves.

NREM-1

NREM-2

NREM-3

REM

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Quiz 2

Match the sleep stages to their descriptions by dragging each colored circle to the appropriate gray circle. When all the circles have been placed, select the CHECK ANSWER button.

Select the NEXT button and move to the Conclusion.

Perhaps you should go back to review the stages of sleep.

NREM-1

NREM-2

NREM-3

REM

stage with moderately slow waves interspersed with bursts of rapid activity

stage of sleep in which the brain and eyes are highly active, and vivid dreams occur

stage of light sleep; transition between wakefulness and sleep

stage of deep sleep consisting almost entirely of large slow waves

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The figure includes a woman sleeping. She has electrodes on her forehead, which are connected to an EEG machine. The readout from the EEG machine appears at the top of the page and is labeled as EEG wave tracing. The waves start as small frequent spikes in NREM-1. The waves become taller during NREM-2. Then they become wider and taller in NREM-3. The waves get smaller as they enter NREM-2 again. Then the waves become even smaller in REM.

Congratulations!

You have completed the activity Stages of Sleep.