Introduction to Sleep and Polysomnography

James D. Geyer, MD

Paul R. Carney, MD

Overview of Sleep Stages and Cycles

The monitoring of sleep is complex and requires a distinct skill set including a detailed knowledge of electroencephalographic (EEG), respiratory monitoring and EKG. Expertise in only one of these areas does not confer the ability to accurately interpret a polysomnogram. On the contrary, understanding only one facet of recording can engender a false sense of mastery.

Sleep is not homogeneous. It is quite heterogeneous and divided into various stages based on EEG, electrooculographic (EOG) or eye movements, and electromyographic (EMG) activity (1,2,3). The basic terminology and methodology involved in monitoring each type of activity will be reviewed in detail. Sleep is composed of nonrapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep is further divided into stages N1, N2, and N3. Stages N1 and N2 are called light sleep, and stage N3 is called deep, delta, or slow-wave sleep. There are usually four or five cycles of sleep in a typical night, each composed of a segment of NREM sleep followed by REM sleep. Periods of wake may also interrupt sleep during the night but should be brief and self-limited in the normal adult. As the night progresses, the length of the REM sleep period in each cycle usually increases. The hypnogram is a convenient method of graphically displaying large amounts of information about the organization of sleep. Each stage of sleep is characterized by a level on the vertical axis of the graph with time of night on the horizontal axis.

Because sleep monitoring was traditionally recorded by paper polygraph recording systems, the night was divided into epochs of time that corresponded to the length of each paper page. Based on the standard paper speed for sleep recording of 10 mm per second, a 30-cm page represented 30 seconds. Each 30-second page was referred to as an epoch. Though modern polysomnography is performed digitally, the convention of scoring sleep in 30-second epochs remains the standard. When a shift in sleep stage occurs during a given epoch, the stage present for the majority of that epoch defines the stage scored for that epoch.

Sleep Architecture Definitions

The term sleep architecture describes the structure of sleep. Common terms used in sleep monitoring are listed in Table 1-1. 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 time spent in sleep stages N1, N2, N3, and R 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) and before the final awakening of the study; 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. Stages N1 to N3 and R can also be displayed as a percentage of total sleep time (%TST). Alternatively, the various sleep stages and WASO can be calculated 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.

TABLE 1-1 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
Stage N1, 2, 3, and REM as %TST—percentage of TST occupied by each sleep stage
Stage N1, 2, 3, and REM, WASO as %SPT—percentage of SPT occupied by sleep stages and WASO
Arousal index

TABLE 1-2 Representative Changes in Sleep Architecture

	20-Year-Old	60-Year-Old	Severe Sleep Apnea^a
WASO% SPT	5	15	20
Stage N1% SPT	5	5	10
Stage N2% SPT	50	55	60
Stage N3% SPT	20	5	0
Stage R% SPT	25	20	10
^aHigh interpatient variability.

The normal range of the percentage of sleep spent in each sleep stage varies with age (2,3) and is affected by sleep disorders (Table 1-2). In adults there is a decrease in stage N3 sleep with increasing age, while the amount of REM sleep remains fairly constant over time. The changes in stage N3 are related in part to a lowering of the voltage of the delta activity secondary to the filtering effects of the skull with advancing age. The amounts of stage N1 sleep and WASO also increases with age. Severe obstructive sleep apnea (OSA) can result in dramatically diminished amounts of stage N3 and stage R. Chronic insomnia (difficulty initiating or maintaining sleep) is characterized by a long sleep latency or increased WASO. The amount of time in stages N3 and R sleep is commonly decreased as well. The REM latency is also affected by sleep disorders and medications. A short REM latency (usually <70 minutes) may occur in some cases of sleep apnea, depression, narcolepsy, prior REM sleep deprivation, or 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, or any significant sleep disturbance.

Electroencephalographic Terminology and Monitoring

Standard sleep monitoring requires monitoring of frontal, central, and occipital EEG activity (preferably bilaterally), which are referenced to mastoid electrodes. Alpha activity is more prominent
in occipital tracings. The terminology for the electrodes adheres to the International 10-20 nomenclature, which guides the exact placement of the electrodes. Even subscripts refer to electrodes on the right and odd subscripts to electrodes on the left side of the head. The usual derivations use the recording electrodes referenced to the opposite mastoid electrode (e.g., C4-M1). Use of greater interelectrode distances increases the voltage difference between electrodes, yielding more easily distinguished waveforms. Full EEG montage recording enhances the ability to identify EEG abnormalities.

EEG activity is characterized by the signal amplitude (voltage), direction of major deflection (polarity), and frequency in cycles per second or hertz (Hz). Standard frequency ranges include delta (<4 Hz), theta (4 to 7 Hz), alpha (8 to 13 Hz), and beta (>13 Hz). Alpha activity is commonly noted during relaxed wakefulness with eyes closed. This activity is best recorded from the occipital derivations but may spread temporally in some individuals. Alpha activity should attenuate when the eyes are open. Bursts of alpha waves also are seen during brief arousals from sleep. Alpha activity can also be seen during REM sleep but is typically several Hz slower than the alpha activity recorded during wakefulness. Alpha activity decreases with the onset of stage N1 sleep. Vertex sharp waves are high-amplitude, centrally predominant negative waves (upward deflection on EEG tracings) with a short duration occurring during the transition from Stage N1 to N2 sleep. A sharp wave is defined as deflection lasting between 70 and 200 milliseconds (Table 1-3).

TABLE 1-3 Standard Sensitivity and Filter Settings

	Sensitivity	Low Filter	High Filter
EEG	50 µV = 1 cm; 100 µV = 1 channel width	0.3^a	35^a
EOG	50 µV = 1 cm; 100 µV = 1 channel width	0.3	35
EMG	50 µv = 1 cm; 100 µV = 1 channel width	10	100
EKG		0.3	70
Airflow (thermistor)	Variable	0.1	15
Chest	Variable	0.1	15
Abdomen	Variable	0.1	15
Sao₂ (%)	1 Volt = 0%-100% or 50%-100%	DC	15
Nasal pressure machine flow	Variable	DC or AC with low filter setting of 0.03	15
			100 to see snoring
PAP flow		DC	DC
Snoring		10	10
^aNote that these filter settings are different from traditional EEG monitoring settings.

Sleep spindles, which are characteristic of stage N2 sleep, are sinusoidal oscillations of 12 to 14 Hz with a duration lasting between 0.5 and 1.5 seconds. They may persist into stage N3 but usually do not occur in stage R. The K complex is a high-amplitude, biphasic wave lasting at least 0.5 second and consisting of an initial sharp, negative voltage followed by a positive deflection 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 waves or delta activity appears. These are high-amplitude, broad waves. In contrast to the EEG definition of delta activity as less than 4 Hz, delta slow-wave activity is defined for sleep staging purposes as waves less than 2 Hz (longer than 0.5-second duration waveforms) with a peak-to-peak amplitude of greater than 75 µV and is measured utilizing the frontal derivations (1). Because a K complex resembles slow-wave activity, differentiating the two can be challenging in some instances. However, by definition, a K complex should stand out or be distinct from the lower-amplitude, background EEG activity. Therefore, a continuous
series of high-voltage slow (HVS) waves would not be considered to be a series of K complexes.

Sawtooth waves are notched-jagged waves of frequency in the theta range (3 to 7 Hz) that may occur during stage R sleep. These waveforms are not required for stage R, but their presence can help confirm its presence.

Eye Movement Recording

Eye movement recording is possible because an electrical potential difference exists across the eyeball: front positive (+), back negative (-). Two things can confound this standardized approach: asymmetric or dysconjugate eye movements and an artificial eye.

There are two common patterns of eye movements. Slow eye movements (SEMs), also called slow-rolling eye movements, are pendular, oscillating movements seen in drowsy (eyes closed) wakefulness and stage N1 sleep. By stage N2 sleep, SEMs usually have disappeared. REMs are sharper (narrower deflections), which are typical of eyes-open wake and REM sleep. Reading eye movements occur with a slow movement in one direction followed by a fast recovery in the other direction. These movements are usually rhythmic and recurring.

In the two-tracing method of eye movement recording, largeamplitude EEG activity or artifact reflected in the EOG tracings usually causes in-phase defections. This is commonly seen with K complexes and stage N3 sleep delta activity.

The main purpose of recording eye movements is to identify the scanning or reading eye movements of wakefulness, slow rolling eye movements of drowsiness, and 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-M1 and LOC-M2). In pediatric recording, alternative eye leads referenced to FPz may be more easily tolerated (4,5).

The electric dipole of the eye, with the cornea being positive in relation to the retina, allows for the recording of eye movements with surface electrodes (4,6). Eye movements are typically conjugate, with both eyes moving toward one eye electrode and away from the other.

High-amplitude EEG activity or artifact occurring in the EOG tracings usually causes in-phase defections and not the out-of-phase deflections seen with conjugate eye movements.

Electromyographic Recording

Usually, three EMG leads are placed in the mental and submental areas. The voltage between two of these three is monitored (e.g., 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 an essential element only for identifying stage R sleep. In stage R, the chin EMG is relatively reduced, with the amplitude being equal to or lower than the lowest EMG amplitude in NREM sleep. The chin EMG may also reach the REM level long before the onset of REMS or an EEG meeting criteria for stage R. Depending on the gain, a reduction in the chin EMG amplitude from wakefulness to sleep and often a further reduction on transition from stage N1 to N3 may be seen. However, a reduction in the chin EMG is not required for stages N2 to N3. The reduction in the EMG amplitude during REM sleep is a reflection of the generalized skeletal-muscle hypotonia present in this sleep stage. Brief EMG activity bursts, referred to as phasic activity, may be seen during REM sleep, especially when there is vigorous eye movement. The combination of REMs, a relatively reduced chin EMG, and a low-voltage mixed-frequency EEG is consistent with stage R.

Sleep Stage Characteristics

The basic rules for sleep staging are summarized in Table 1-4. Note that some characteristics are required and some are helpful but not required to stage a particular epoch. 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, and the chin EMG activity is relatively high, allowing differentiation from Stage R sleep. During eyes-closed drowsy wake,
the EEG is characterized by prominent alpha activity (>50% of the epoch). Both slow, scanning and rapid irregular eye movements are usually present. The level of muscle tone is usually relatively high. The epoch should be scored as stage W when more than 50% of the epoch consists of alpha rhythm or findings consistent with stage W, such as eye blinks (conjugate vertical eye movements with a frequency between 0.5 and 2 Hz), reading eye movements (series of repetitive movements with a slow phase followed by a rapid or return phase), or REMs with a high chin EMG tone. Caution should be used in scoring stage W with REMs and a high chin EMG tone since this may also occur in REM sleep behavior disorder (RBD).

TABLE 1-4 Summary of Sleep Stage Characteristics

	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 N1	Low-amplitude mixed-frequency <50% Alpha activity No spindles, K complexes	Slow-rolling eye movements	May be lower than wake
	Sharp waves near transition to stage N2
Stage N2	At least one sleep spindle or K complex <20% Slow-wave activity^b		May be lower than wake
Stage N3	>20% Frontally predominant slow-wave activity	^c	Usually low
Stage R	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. ^cSlow waves usually seen in EOG tracings.

The alpha rhythm is composed of 8- to 13-Hz waves over the posterior head regions during relaxed wakefulness with eyes closed. The lower limit of 8 Hz is typically attained by 8 years of age. The frequency of the alpha rhythm in adults is typically between 9 and 11 Hz, decreasing slightly with advancing age. A posteriorly dominant rhythm of less than 8 Hz during wakefulness in an adult is abnormal. Identifying the waking background activity is vital for correct sleep staging. The frequency and morphology of the alpha rhythm should be similar over the two hemispheres. An interhemispheric asymmetry of the alpha rhythm of 1 Hz or greater is also abnormal. Importantly, in some individuals, no waking alpha rhythm can be identified. The individual may have a low-amplitude fast background during wakefulness.

In referential montages, the distribution of the alpha rhythm is usually maximal at the occipital electrodes (O1, O2). In some cases, the amplitude of the alpha rhythm may be highest in the parietal or posterior temporal regions and occasionally is more diffusely distributed. The voltage of the alpha rhythm in adults is in the range of 15 to 45 µV. Higher voltages are observed in younger individuals. The voltage decreases with advancing age, secondary to changes in bone density and increased electrical impedance of intervening tissue. A mild voltage asymmetry is common, with the right hemisphere typically being of a somewhat higher amplitude. Voltage asymmetries are considered significant when the interhemispheric amplitude difference is greater than 50%.

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