Answer
SWS, or delta sleep, is characterized, as the name implies, by delta activity. This is typically generalized and polymorphic or semirhythmic. By strict sleep staging criteria on polysomnography, SWS is defined by the presence of such delta activity for more than 20% of the time, and an amplitude criterion of at least 75 µV is often applied.
The distinction between stage III and stage IV sleep is only a quantitative one that has to do with the amount of delta activity. Stage III is defined by delta activity that occupies 20-50% of the time, whereas in stage IV, delta activity represents greater than 50% of the time. Sleep spindles and K complexes may persist in stage III and even to some degree in stage IV, but they are not prominent.
Clinical correlation
As already mentioned, SWS is usually not seen during routine EEG, which is too brief a recording. However, it is seen during prolonged EEG monitoring. One important clinical aspect of SWS is that certain parasomnias occur specifically out of this stage and must be differentiated from seizures. These slow wave sleep parasomnias include confusional arousals, night terrors (pavor nocturnus), and sleepwalking (somnambulism).
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The earliest indication of transition from wakefulness to stage I sleep (drowsiness) is shown here and usually consists of a combination of (1) drop out of alpha activity and (2) slow rolling eye movements.
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Slow rolling (lateral) eye movements during stage I sleep. Like faster lateral eye movements, slow ones are best seen at the F7 and F8 electrodes, with the corneal positivity indicating the side of gaze.
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On this transverse montage, typical vertex sharp transients are seen. In contrast to K complexes, these are narrow (brief) and more focal, with a maximum negativity at the mid line (Cz and to a lesser degree Fz). These are seen in sleep stages I and II.
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Vertex waves are focal sharp transients typically best seen on transverse montages (through the midline) and would be missed on this longitudinal bipolar montage if it did not include midline channels (Fz-Cz-Pz). Vertex waves are seen in sleep stages I and II.
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Positive occipital sharp transients of sleep (POSTS) are seen in both occipital regions, with their typical characteristics contained in their name. They also have morphology classically described as "reverse check mark" and often occur in consecutive runs of several seconds, as shown here.
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This shows a K complex, typically a high-amplitude long-duration biphasic waveform with overriding spindle. This is a transverse montage, which shows the typical maximum (manifested by a "phase reversal") at the midline.
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Typical sleep spindles with short-lived waxing and waning 15-Hz activity maximum in the frontocentral regions. Note the associated slow (theta) activity that also characterizes stage II sleep.
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Vertex sharp transients. This transverse montage illustrates the maximum negativity (manifested by a negative phase reversal) at the midline. The location is similar to that of K complexes, but these are shorter (narrower) and more localized.
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K complex, with its typical characteristics: high-amplitude, widespread, broad, diphasic slow transient with overriding spindle. On the longitudinal montage (left), the K complex appears to be generalized. However, the transverse montage clearly shows that the maximum (phase reversal) is at the midline (Fz and Cz).
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A mixture of spindles (ie, bicentral short-lived rhythmic 14 Hz bursts) and positive occipital sharp transients of sleep (POSTS) can be seen. POSTS occur in stage I, but the presence of spindles is "diagnostic" of stage II.
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A mixture of positive occipital sharp transients of sleep (POSTS) and spindles (fronto-central short-lived rhythmic 14-Hz bursts) can be seen.
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Slow wave sleep with predominantly delta activity, especially in the first half.
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Slow wave sleep with predominantly delta activity.
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Rapid eye movement sleep with rapid (saccadic) eye movements. While muscle "atonia" cannot be proven without a dedicated electromyogram (EMG) channel, certainly EMG artifact is absent with a "quiet" recording. Also, no alpha rhythm is present that would suggest wakefulness.
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Typical saccadic eye movements of rapid eye movement sleep are shown, with lateral rectus "spikes" seen just preceding the lateral abducting eye movements.
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In addition to rapid eye movements, this rapid eye movement sleep record is characterized by brief fragments of alpha rhythm (first half) and central saw tooth waves (second half).
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This is a good example of saw tooth waves seen in rapid eye movement sleep and their "notched" morphology.
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This is a good example of saw tooth waves seen in rapid eye movement sleep and their "notched" morphology, best seen here in the Cz-Pz (last) channel.
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This illustrates the typical appearance of saw tooth waves on a polysomnogram (PSG) display, equivalent to 1 cm/s.
