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Sleep Heart Health Study

6.6.2.6 Rules for assigning sleep stages

EEG frequencies are divided into following bandwidths:

           β  (beta)     > 13 Hz
13 Hz    ≥ α  (alpha)    ≥  8 Hz
 8 Hz    > θ  (theta)    ≥  4 Hz
 4 Hz    > δ  (delta)

An alpha wave is any wave that has the frequency in alpha range. Alpha rhythm (also known as posterior background rhythm) has the following characteristics:

  • Is seen in the relaxed waking state with the eyes closed.
  • Attenuates with eye opening, anxiety or mental activity such as mental calculations.
  • Slows in drowsiness (occasionally <8 Hz) and then disappears in sleep. The slowing may be so brief as to be unnoticed.
  • Generated by occipital lobes and has a broad reflection to temporal and mastoid areas.

An illustration of the differences in alpha with open and closed eyes is shown in (SsFigures 1a, 1b, 1c, and 1d).

Stage W - Waking State

Stage W, when eyes are open, is defined by low voltage, mixed frequency EEG in the alpha and beta ranges (> 8 Hz). When eyes are closed, wake is defined by the presence of the alpha rhythm. There is usually (but not necessarily), a relatively high tonic EMG. Waking shows frequent eye movements and eye blinks. Some subjects may have virtually continuous alpha activity, others may show little or no alpha activity in the waking record.

Stage 1 sleep

Stage 1 sleep occurs most often in transition from wakefulness to other sleep stages.

Stage 1 is defined by a background of relatively low voltage, mixed frequency EEG activity with noticeable activity in the 2-7 Hz range with no clearly defined K complexes or sleep spindles. Faster frequencies are mostly lower voltage (amplitude). High voltage (50-75 μV) 2-7 Hz activity tends to occur in irregularly spaced bursts mostly during the later portions of the stage. There are slow eye movements, each of several seconds duration, usually most prominent during early portions of the stage. No rapid eye movements or blinks are present. During the latter portion of the stage, vertex sharp waves, occasionally as high as 200 μV, are often seen in conjunction with high amplitude 2-7 Hz activity. The amount of alpha activity combined with low voltage activity comprises less than half of the epoch. Finally, the tonic EMG level may be lower than observed during relaxed wakefulness. (SsFigures 2a, 2b, 2c, and 2d)

Traces of low voltage activity at 12-14 Hz may begin to appear as the transition to Stage 2 approaches, but this activity is not defined as sleep spindles until the rhythmic bursts are clearly visible for at least 0.5 s.

Stage 2 sleep

Stage 2 is defined by a background similar to Stage 1 sleep with the presence of the K complexes and/or sleep spindles. It is impossible to define the difference between Stage 1 and Stage 2 sleep on the basis of background activity alone. Bursts of other polymorphic high voltage slow waves, which do not have the precise morphology of K complexes, are also frequently seen. Delta waves: high amplitude ( > 75 μV), slow (0.5-2 Hz; duration 0.5-2 sec) activity occupy no more than 19% of the epoch. At the beginning of the Stage 2, slow eye movements may infrequently, and only briefly, persist after the appearance of sleep spindles and K complexes. (SsFigures 3a, 3b, and 3c)

The "3 minute rule" was not followed. Once stage 2 sleep was achieved, stage 2 sleep was continued to be scored unless there was a transition to deep sleep, REM, or wake.

K complexes may be imbedded within an arousal and nonetheless constitute evidence of Stage 2. (SsFigure 4a)

Deep Sleep (Stage 3 and 4)

In SHHS, no attempt is made to distinguish Stage 3 from Stage 4 which are combined into a single category: Deep Sleep.

Deep Sleep is scored when 20% or more of the epoch consists of delta waves which are 0.5-2 Hz (duration 0.5-2 sec) and have an amplitude greater than 75 μV. The 20% criteria refers specifically to the time occupied by the high amplitude, slow waves, and does not include intervening waves of higher frequency and lower amplitude or K complexes. To fulfill the criteria for Deep Sleep, one should be able to find at least 5-6 high voltage delta waves in the 30 second sleep epoch (SsFigures 5a and 5b). Delta waves embedded in increased frequency activity (an arousal) do not contribute to the calculation of time in delta sleep. (SsFigure 5c)

Sleep spindles and K complexes may or may not be present in Deep Sleep (SsFigure 5d). Eye movements do not occur in Deep Sleep, although the EOG may reflect the high voltage slow wave activity. The EMG is tonically active, although the tracing may achieve very low levels, indistinguishable from that of REM sleep.

Note: In the Compumedics software Stages 2, 3/4 are assigned based on the amount of the “delta-H” waves. K complexes and all the waves from the delta range (< 2 Hz) are considered by the computer algorithm in the count of these waves. The computer based calculation of delta sleep can be used to help determine % of slow waves when there are K complexes or events or other artifact. When any ambiguity exists, the scorer must quantify the percentage of time delta waves occupy within the epoch. Any questionable areas should be re-measured, subtracting time occupied by K complexes and other waveforms.

An attempt should be made to distinguish between spontaneous K complexes and delta waves, although this distinction is not always easy. When a K complex distinction is in doubt, comparison should be done with the K complex in unambiguous Stage 2.

Stage REM sleep

Stage REM is defined by a background of relatively low voltage, mixed frequency EEG with accompanying episodes of REMs (Rapid Eye Movements). The EEG pattern resembles Stage 1, except that vertex sharp waves are not readily noticeable. Bursts of characteristic “sawtooth” waves may appear, appearing as notched waves in the theta range. Alpha activity is usually more prominent than in Stage 1 and its frequency is 1-2 Hz slower than the alpha rhythm in wakefulness. The EMG reaches its lowest levels (it cannot be higher than the level during the preceding stage). Phasic twitches and intermittent increases of EMG activity may be observed but intervening baseline must remain low. Phasic twitch (EMG) defined as: short (no longer than .10 sec) burst of EMG activity superimposed on suppressed muscle tone which physically manifests as a twitch (contraction) of a muscle or jerk of a limb. In REM such muscle contractions may be isolated or become repetitive, but they remain distinctive. Periods of the relatively low voltage, mixed frequency EEG and EMG at Stage REM level but without eye movements may follow unambiguous stage REM and is considered Stage REM unless criteria for a state change are met. (SsFigures 6a, 6b, 6c, 6d, and 6e)

The EOG shows bursts of rapid eye movements; often the density of such bursts increase as sleep progresses. Thus, earlier Stage REM episodes usually contain fewer REMs than later episodes.

Rarely delta waves may be observed in an epoch that is within a period of REM (SsFigures 7a and 7b). If occurring within period of REM, a low EMG, continue to score as REM. Large sawtooth waves also may be confused with hypersyncrony (SsFigure 7c).

Note: Excessive beta activity may be observed in REM and should not be confused with spindles. Medications (benzodiazepine or barbiturate ingestion) may induce excessive beta activity in both REM and Non-REM sleep. This beta activity can mimic sleep spindles. Their frequencies often are faster than those seen with the true sleep spindles (see above Stage 2 sleep section for guidance on identifying spindles). Rarely, sleep spindles can be seen in REM in subjects with substantial sleep deprivation ([SsFigures](:pages_path:/mop/6-AA-mop-list-of-illustrations.md) 7d, 7e, and 7f).

Start of the Stage REM:

At the start of Stage REM, K complexes, sleep spindles and delta waves end, characteristic sawtooth waves can appear. EMG levels tend to be the lowest after eye movements begin. The fall in EMG may not coincide with the EEG changes.

  • If the EMG drops before the last sleep spindle, K complex or delta wave: Score Stage REM from the point the last sleep spindle, K complex or delta wave was seen. (SsFigure 6a).
  • Otherwise: Score Stage REM from the point where EMG drops. The period of the record before the EMG drop is scored according to the rules for NREM sleep. (SsFigure 8a).

Periods of elevated sustained EMG during Stage REM sleep:

When EMG is elevated above the REM level for longer than 15 sec, then this portion of the record is scored as a Non-REM sleep or WAKE. If phasic twiches or sawtooth waves are seen, but intervening EMG is low, the epoch remains REM (SsFigure 6e).

WAKE-REM Transition:

When one epoch or more of Stage Wake slows into a low voltage, mixed EEG pattern before REMs or sawtooth waves begin: Score Stage 1 until the EMG drops to REM level and Stage REM afterwards. (*) (SsFigure 8a)

When Stage Wake (> 15s.) interrupts Stage REM (i.e., REM-> WAKE ): (*)

If the EEG background of the epochs between the intervening waking epoch and the appearance of REMs (eye movements) is ambiguous (i.e. the EEG background is compatible with either REM or Non-REM sleep), then Score as Stage 1 if the EMG is elevated (SsFigure 8a). Score as Stage REM when EMG is at the Stage REM level (SsFigures Series 9a, 9b, 9c, 9d, and 9e). However, if a sleep spindle or K-complex appears prior to the appearance of REMs, then the waking epochs are considered the end of the Stage REM (SsFigures Series 9f, 9g, 9h, 9i, 9j, and 9k).

When arousals (< 15 sec) interrupt REM:

The occurrence of an arousal in REM does not automatically change sleep state. However, if the EMG increases after the arousal, REM is considered terminated by the arousal. The following epoch is non REM.

Note: Often a single K complex may be seen in REM after an arousal, with subsequent REMs. This per se does not change REM state. In particular, K complexes do not change REM state if intervening eye movements are seen following the K complex or the interval between the first K complex and the subsequent K complex is > 3 minutes ([SsFigure](:pages_path:/mop/6-AA-mop-list-of-illustrations.md) 7f). In contrast, a K-complex occurring after a period of wake interrupting REM does change the state (beginning with Stage 2 at the first K-complex, and Stage 1 between the end of wake and the next K-complex) ([SsFigures](:pages_path:/mop/6-AA-mop-list-of-illustrations.md) 9f, 9g, 9h, 9i, 9j, and 9k).

End of the Stage REM

Stage REM is scored until a clear evidence of sleep stage change is visualized:

  • Appearance of K complexes or sleep spindles without presence of eye movements or sawtooth waves between them, or at intervals of < 3 minutes (SsFigures 10a, 10b, 10c, and 10d).
  • Transition to Stage Wake (SsFigures 9b and 9c).
  • Sustained, increased EMG (SsFigures 11a, 11b, 11c, and 11d).

When K complexes or unquestionable sleep spindles are seen in the Stage REM:

An isolated K complex may be seen in REM. When K complexes or unquestionable sleep spindles (as compared to Stage 2) are present in stage REM, then an interval between two K complexes or sleep spindles is scored as Stage 2 only if there are no REMs (eye movements) or sawtooth waves in this interval. Otherwise the interval is scored as Stage REM (SsFigure 7f).

Scoring REM when there are problems with REM related atonia or the EMG is difficult to interpret.

Identification of Stage REM may be difficult when there are prolonged bursts of elevated EMG seen during eye movements or the EMG increases with snoring. In such cases:

  • EMG increases clearly related to snoring (changing with breathing) may be ignored, as long as intervening EMG is low (SsFigure 12a).
  • The portion of the record with unquestionable Stage REM should be reviewed to provide a visual reference of the characteristic Stage REM EEG pattern. Stage REM is scored when EEG pattern changes to the pattern characteristic for Stage REM regardless of the level of the EMG (SsFigures 12b and 12c). When EEG is consistent with Stage REM, and there is no evidence of wake (blinking), no evidence of Stage 1 (vertex waves, slow rolling eye movements), or the presence of REMs, then score stage Stage REM from the last K complex, sleep spindles or delta wave. Distinguishing between Stage 1 and Stage REM is unreliable and noted as such on the PSG scoring notes form.


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Sleep Heart Health Study