Introduction to Sleep and Polysomnography

James D. Geyer

Paul R. Carney

OVERVIEW OF SLEEP STAGES AND CYCLES

The monitoring of sleep is complex and requires a distinct skill set including a detailed knowledge of EEG, respiratory monitoring, and EKG. Expertise in only one of these areas does not confer the ability to accurately interpret the polysomnogram.

Sleep is not homogeneous. It is characterized by sleep stages based on electroencephalographic (EEG) or electrical brain wave activity, electro-oculographic (EOG) or eye movements, and electromyographic (EMG) or muscle electrical activity.¹^,²^,³ The basic terminology and methods involved with monitoring each of these types of activity are discussed below. Sleep is composed of nonrapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is further divided into stages 1, 2, and 3. Stages 1 and 2 are called light sleep and stage 3 is called deep or slow-wave sleep. There are usually four or five cycles of sleep per night, each composed of a segment of NREM sleep followed by REM sleep. Periods of wake may also interrupt sleep. As the night progresses, the length of REM sleep in each cycle usually increases. The hypnogram (Fig. 18.1) is a convenient method of graphically displaying the organization of sleep during the night. Each stage of sleep is characterized by a level on the vertical axis of the graph with time of night on the horizontal axis. REM sleep is often highlighted by a dark bar.

Sleep monitoring was traditionally by polygraph recording using ink-writing pens that produced tracings on paper. It was convenient to divide the night into epochs of time that correspond to the length of each paper page. The usual paper speed for sleep recording is 10 mm/s; a 30-cm page corresponds to 30 seconds. This is three times slower than routine EEG, which classically records at 30 mm/s generating a 10-second page. Each segment of time represented by one page is called an epoch; sleep is staged in epochs. Today most sleep recording is performed digitally, but the convention of scoring sleep in 30-second epochs or windows is still the standard. If there is a shift in sleep stage during a given epoch, the stage present for the majority of the time names the epoch.

SLEEP ARCHITECTURE DEFINITIONS

The term sleep architecture describes the structure of sleep. Common terms used in sleep monitoring are listed in Table 18.1. The total monitoring time or total recording time (TRT) is also called total bedtime (TBT). This is the time duration from lights out (start of recording) to lights on (termination of recording). The total amount of sleep stages 1, 2, 3, and REM, is termed the total sleep time (TST). The time from the first sleep until the final awakening is called the sleep period time (SPT). SPT encompasses all sleep as well as periods of wake after sleep onset (WASO) before the final awakening. Therefore, SPT = TST + WASO. The time from the start of sleep monitoring (or lights out) until the first epoch of sleep is called the sleep latency. The time from the first epoch of sleep until the first REM sleep is called the REM latency. It is useful not only to determine the total minutes of each sleep stage but also to characterize the relative proportion of time spent in each sleep stage. One can characterize stages 1-3 and REM as a percentage of total sleep time (%TST). Another method is to characterize the sleep stages and WASO as a percentage of the sleep period time (% SPT). Sleep efficiency (in percent) is usually defined as either the TST × 100/SPT or TST × 100/TBT.

The normal range of the percentage of sleep spent in each sleep stage varies with age²^,³ and is impacted by sleep disorders (Table 18.2). In adults, there is a decrease in stage 3 sleep with increasing age, while the amount of REM sleep remains fairly constant. The amounts of stage 1 sleep and WASO also increase with age. In patients with severe obstructive sleep apnea (OSA), there is often no stage 3 sleep and a reduced amount of REM sleep. Chronic insomnia (difficulty initiating or
maintaining sleep) is characterized by a long sleep latency and increased WASO. The amount of stages 3 and REM sleep is commonly decreased as well. The REM latency is also affected by sleep disorders and medications. A short REM latency (usually <70 minutes) is noted in some cases of sleep apnea, depression, narcolepsy, prior REM sleep deprivation, and the withdrawal of REM suppressant medications. An increased REM latency can be seen with REM suppressants (ethanol and many antidepressants), an unfamiliar or uncomfortable sleep environment, sleep apnea, and any process that disturbs sleep quality.

FIGURE 18.1. Normal hypnogram: The various stages of sleep are represented by levels on the vertical axis; time of night is shown on the horizontal axis. In this patient, there were four sleep cycles each composed of a segment of NREM sleep followed by REM sleep. The REM sleep increased toward morning, while most of the stage 3 sleep was in the early portion of the night.

TABLE 18.1 Sleep architecture definitions

Lights out: start of sleep recording
Light on: end of sleep recording
TBT (total bedtime): time from lights out to lights on
TST (total sleep time) = minutes of stages 1, 2, 3, and REM
WASO (wake after sleep onset): minutes of wake after first sleep but before the final awakening
SPT (sleep period time) = TST + WASO
Sleep latency: time from lights out until the first epoch of sleep
REM latency: time from first epoch of sleep to the first epoch of REM sleep
Sleep efficiency = (TST × 100)/TBT
Stages 1, 2, 3, and REM as % TST: percentage of TST occupied by each sleep stage
Stages 1, 2, 3, and REM, WASO as % SPT: percentage of SPT occupied by sleep stages and WASO
Arousal index: number of arousals per hour of sleep

TABLE 18.2 Representative changes in sleep architecture

%SPT	20-Year-Old	60-Year-Old	Severe Sleep Apnea^a
WASO	5	15	20
Stage 1	5	5	10
Stage 2	50	55	60
Stage 3	20	5	0
REM	25	20	10
^aHigh interpatient variability.

INTRODUCTION TO ELECTROENCEPHALOGRAPHIC TERMINOLOGY AND MONITORING

EEG activity is characterized by the frequency in cycles per second or hertz (Hz), amplitude (voltage), and the direction of major deflection (polarity). The classically described frequency ranges are delta (<4 Hz), theta (4-7 Hz), alpha (8-13 Hz), and beta (>13 Hz). Alpha waves (8-13 Hz) are commonly noted when the patient is in an awake, but relaxed, state with the eyes closed. They are best recorded over the occiput and are attenuated when the eyes are open. Bursts of alpha waves also are seen during brief awakenings from sleep—called arousals. Alpha activity can also be seen during REM sleep. Alpha activity is prominent during drowsy eyes-closed wakefulness. This activity decreases with the onset of stage 1 sleep. Vertex waves may appear near the transition from stage 1 to stage 2 sleep. Vertex waves are negative, shortduration, high-amplitude sharp waves. They are more prominent in central than in occipital EEG tracings. A sharp wave is defined as a sharply contoured deflection of 70-200 ms in duration.

Sleep spindles are oscillations at 12-14 Hz with a duration of 0.5-1.5 seconds. They are characteristic of stage 2 sleep. They may persist into stage 3, but usually do not occur in stage REM. The K complex is a high-amplitude, biphasic wave of at least a 0.5-second duration. As classically defined, a K complex consists of an
initial sharp, negative voltage (by convention an upward deflection) followed by a positive-deflection (down) slow wave. Spindles frequently are superimposed on K complexes. Sharp waves differ from K complexes in that they are narrower, not biphasic, and usually of lower amplitude.

As sleep deepens, slow (delta) waves appear. These are high-amplitude, broad waves. In contrast to the EEG definition of delta activity as <4 Hz, delta slow-wave activity is defined for sleep staging purposes as waves slower than 2 Hz (longer than 0.5-second duration) with a peak-to-peak amplitude of >75 µV. The amount of slow-wave activity as measured in the central EEG derivations is used to determine if stage 3 is present¹ (see below). Because a K complex resembles slow-wave activity, differentiating the two is sometimes difficult. However, by definition, a K complex should stand out (be distinct) from the lower-amplitude, background EEG activity. Therefore, a continuous series of high-voltage slow waves would not be considered to be a series of K complexes.

Sawtooth waves are notched-jagged waves with frequency in the theta range (3-7 Hz) that may be present during REM sleep. Although they are not part of the criteria for scoring REM sleep, their presence is a clue that REM sleep is present.

EYE MOVEMENT RECORDING

The main purpose of recording eye movements is to identify REM sleep. EOG (eye movement) electrodes typically are placed at the outer corners of the eyes—at the right outer canthus (ROC) and the left outer canthus (LOC). In a common approach, two eye channels are recorded, and the eye electrodes are referenced to the opposite mastoid (ROC-A1 and LOC-A2). However, some sleep centers use the same mastoid electrode as a reference (ROC-A1 and LOC-A1). To detect vertical as well as horizontal eye movements, one electrode is placed slightly above and one slightly below the outer canthus of each eye.⁴^,⁵

Recording of eye movements is possible because a potential difference exists across the eyeball: front positive (+) and back negative (-). Eye movements are detected by EOG recording of voltage changes. When the eyes move toward an electrode, a positive voltage is recorded (see Fig. 18.2). By standard convention, polygraphs are calibrated so that a negative voltage causes an upward pen deflection (negative polarity up). Thus, eye movement toward an electrode results in a downward deflection.⁴^,⁶ Note that movement of the eyes is usually conjugate, with both eyes moving toward one eye electrode and away from the other. If the eye channels are calibrated with the same polarity settings, eye movements produce out-of-phase deflections in the two eye tracings (eg, one up and one down). Figure 18.2 shows the recorded results of eye movements to the right and left (assuming both amplifier channels have negative polarity up). The same approach can be used to understand the tracings resulting from vertical eye movements. Because ROC is positioned above the eyes (and LOC below), upward eye movements are toward ROC and away from LOC. Thus, upward eye movement results in a downward deflection in the ROC tracing and an upward deflection in the LOC tracing.

There are two common patterns of eye movements (Fig. 18.3). Slow eye movements (SEMs), also called slow-rolling eye movements, are pendular oscillating movements that are seen in drowsy (eyes closed) wakefulness and stage 1 sleep. By stage 2 sleep, SEMs usually have disappeared. REMs are sharper (more narrow deflections), which are typical of eyes-open wake and REM sleep.

FIGURE 18.2. The effects of conjugate lateral eye movements on deflections in tracings from ROC (right outer canthus) and LOC (left outer canthus) linked to ipsilateral ear. The front of the globe is positive with respect to the back. Patient looks left (A) or right (B). See Figure 5.4 for effects on frontal EEG electrodes.

FIGURE 18.3. Typical patterns of eye movements in PSG recording. Top two traces (blue) are epicanthal electrodes. A. Slow eye movements (SEMs) are pendular and common in drowsy wake and stage 1 sleep.

FIGURE 18.3. B. Rapid eye movements during phasic REM sleep (most obvious in the last 8 seconds) are sharper and shorter duration. REMs can also be seen as saccadic eye movements in wake with eyes open.

In the two-tracing method of eye movement recording, large-amplitude EEG activity or artifact reflected in the EOG tracings usually causes in-phase deflections.

ELECTROMYOGRAPHIC RECORDING

Usually, three EMG leads are placed in the mental (chin) and submental areas. The voltage between two of these three is monitored (eg, EMG1-EMG3). If either of these leads fails, the third lead can be substituted. The gain of the chin EMG is adjusted so that some activity is noted during wakefulness. The chin EMG is essential, but only for identifying stage REM sleep. In stage REM, the chin EMG is relatively reduced—the amplitude is equal to or lower than the lowest EMG amplitude in NREM sleep. If the chin EMG gain is adjusted high enough to show some activity in NREM sleep, a drop in activity is often seen on transition to REM sleep. The chin EMG may also reach the REM level long before the onset of REMs or an EEG meeting criteria for stage REM. Depending on the gain, a reduction in the chin EMG amplitude from wakefulness to sleep and often a further reduction on transition from stage 1 to 3 may be seen. However, a reduction in the chin EMG is not required for stages 2-3. The reduction in the EMG amplitude during REM sleep is a reflection of the generalized skeletal muscle hypotonia present in this sleep stage. Phasic brief EMG bursts still may be seen during REM sleep. The combination of REMs, a relatively reduced chin EMG, and a low-voltage mixed-frequency EEG is consistent with stage REM.

SLEEP STAGE CHARACTERISTICS

The basic rules for sleep staging are summarized in Table 18.3. Note that some characteristics are required (bold) and some are helpful but not required. The typical patterns associated with each sleep stage are discussed below.

Stage Wake

During eyes-open wake, the EEG is characterized by high-frequency low-voltage activity. The EOG tracings typically show REMs associated with saccades, and the chin EMG activity is relatively high (Fig. 18.4) allowing differentiation from stage REM sleep. During eyes-closed drowsy wake, the EEG is characterized by prominent 8-13 Hz sinusoidal alpha activity, typically most prominent over the occipital region (>50% of the epoch). Eye blinks consisting of conjugate vertical eye movements at a frequency of 0.5-2 Hz and reading eye movements (consisting of trains of slow rightward conjugate eye movements followed by a rapid leftward phase as the individual reads each line) are common during wakefulness. REMs consist of conjugate, irregular, sharply peaked eye movements with an initial deflection usually

lasting <500 ms. They occur with eyes open as individuals visually scan the environment. SEMs are conjugate, sinusoidal eye movements with an initial deflection that usually lasts >500 ms. The level of muscle tone is usually relatively high.

TABLE 18.3 Summary of sleep stage characteristics

Stage	Characteristics^a,^b
Stage	EEG	EOG	EMG
Wake (eyes open)	Low-voltage, high-frequency, attenuated alpha activity	Eye blinks, REMs	Relatively high
Wake (eyes closed)	Low-voltage, high-frequency >50% alpha activity	Slow-rolling eye movements	Relatively high
Stage 1	Low amplitude mixed frequency, <50% alpha activity, NO spindles or K complexes Vertex sharp waves near transition to stage 2	Slow-rolling eye movements	May be lower than wake
Stage 2	At least one sleep spindle or K complex, <20% slow-wave activity^b		May be lower than wake
Stage 3	>20% slow-wave activity	Slow waves^c	Usually low
Stage REM	Low-voltage mixed frequency Sawtooth waves may be present	Episodic REMs	Relatively reduced (equal or lower than the lowest in NREM)
^aRequired characteristics in bold. ^bSlow wave activity, frequency <2 Hz; peak-to-peak amplitude >75 µV; >50% means slow wave activity present in more than 50% of the epoch; REMs, rapid eye movements. ^cCerebral slow waves usually seen in EOG tracings.

FIGURE 18.4. Stage wake-eyes open (30-second tracing). The EEG shows low-amplitude high-frequency activity. The EOG shows blinks and REMs (saccades). The chin EMG is relatively high.

Stage 1

The stage 1 EEG is characterized by low-voltage, mixed-frequency activity (4-7 Hz). Stage 1 is scored when <50% of an epoch contains alpha waves and criteria for deeper stages of sleep are not met (Figs. 18.5 and 18.6). Slow-rolling eye movements are often present in the eye movement tracings, and the level of muscle tone (EMG) is equal or diminished compared to that in the awake state. Some patients do not exhibit prominent alpha activity, making detection of sleep onset difficult. The ability of a patient to produce alpha waves can be determined from biocalibrations at the start of the study. The patient is asked to lie quietly with eyes open and then with the eyes closed. Alpha activity usually appears with eye closure. When patients do not produce significant alpha activity, differentiating wakefulness from stage 1 sleep can be difficult.

Determining the transition to stage 1 sleep is important because this defines sleep onset. Several features are helpful in distinguishing wake from stage 1. First, the presence of REMs in the absence of a reduced chin EMG usually means the patient is still awake. However, SEMs can be present during both drowsy wake and stage 1 sleep. In this case, one must differentiate wake from stage 1 by the EEG. In wake, the EEG has considerable high-frequency activity. In stage 1, the EEG is generally slower with activity in the 4-7 Hz range. Often the easiest method to determine sleep onset in difficult cases is to find the first epoch of unequivocal sleep (usually stage 2) and work backward. By this method, the examiner can usually be confident of the point of sleep onset within one or two epochs.

Vertex waves are common in stage 1 sleep and are defined by a sharp wave maximal over the central derivations, often reversing polarity at the vertex. Vertex waves should be easily distinguished from the background activity.

Stage 2

Stage 2 sleep is characterized by the presence of one or more K complexes (Fig. 18.7) or sleep spindles (Fig. 18.8). K complexes are well-delineated, negative, sharp waves immediately followed by positive components standing out from the background EEG, with total duration ≥0.5 seconds. Sleep spindles are trains of distinct waves with frequency 11-16 Hz (most commonly 12-14 Hz) with a duration ≥0.5 seconds and typically maximal in the central derivations. To qualify as stage 2, an epoch also must contain <20% of slow (delta)-wave EEG activity (<6 seconds of a 30-second epoch). Slow-wave activity is defined as waves with a frequency <2 Hz and a minimum peak-to-peak amplitude of >75 µV. Stage 2 occupies the greatest proportion of the total sleep time and accounts for roughly 40%-50% of sleep. Stage 2 sleep ends with a sleep stage transition (to stage W, stage 3, stage REM), an arousal, or a major body movement followed by SEMs and low-amplitude mixedfrequency EEG.

Stage 3

Stage 3 NREM sleep is called slow-wave, delta, or deep sleep. Stage 3 is scored when slow-wave activity (frequency <2 Hz and amplitude >75 µV peak to peak) is present for >20% of the epoch (Fig. 18.9). Spindles may be present in the EEG. Frequently, the high-voltage EEG activity is transmitted to the eye leads. The EMG often is lower than during stages 1 and 2 sleep, but this is variable. In older patients, the slow-wave amplitude is lower, and the total amount of slow-wave sleep is reduced. The amplitude of the slow waves (and amount of slow-wave sleep) is usually highest in the first sleep cycles. Typically, stage 3 occurs mostly in the early portions of the night. Several parasomnias (disorders associated with sleep) occur in stage 3 sleep and, therefore, can be predicted to occur in the early part of the night. These include somnambulism (sleep walking) and night terrors. In contrast, parasomnias occurring in REM sleep (eg, nightmares) are more common in the early morning hours.

Stage REM

Stage REM sleep is characterized by a low-voltage, mixed-frequency EEG, the presence of episodic REMs, and a relatively low-amplitude chin EMG. REMs consist of conjugate, irregular, sharply peaked eye movements with an initial deflection usually lasting <500 ms. Sawtooth waves consist of trains of sharply contoured or triangular, sawtooth-appearing, 2-6 Hz waves maximal over the central region (see Fig. 18.10).

There usually are three to five episodes of REM sleep during the night, which tend to increase in length as the night progresses. The number of eye movements

per unit time (REM density) also increases during the night. Not all epochs of REM sleep contain REMs. Epochs of sleep otherwise meeting criteria for stage REM and contiguous with epochs of unequivocal stage REM (REMs present) are scored as stage REM (see Advanced Staging Rules). Bursts of alpha waves can occur during REM sleep, but the frequency is often 1-2 Hz slower than during wake.

FIGURE 18.5. Stage wake-eyes closed (drowsy). The EEG shows more alpha activity for more than 50% of the epoch, and the EOG tracing may show slow eye movements. The chin EMG is relatively high in amplitude.

FIGURE 18.6. Stage 1 (30-second tracing). The EEG shows alpha activity for <50% of the epoch and has a low-voltage mixed-frequency activity. Slow eye movements are usually present. Alpha can be seen more anteriorly or replaced by slower theta rhythms.

FIGURE 18.7. A 30-second tracing of stage 2 sleep is shown. The EEG shows a K complex that is transmitted to the eye channels (in-phase deflection marked in green).

FIGURE 18.8. A 30-second tracing of stage 2 sleep is shown. The EEG shows sleep spindles (SS, black arrows), and the eye movement channels show an absence of slow eye movements.

FIGURE 18.9. A 30-second tracing of stage 3 sleep is shown. There is prominent slow-wave activity meeting voltage criteria (>75 µV peak to peak) throughout the tracing. Note that slow-wave activity is also seen in the eye channels.