stage i sleep

Stage I sleep is somewhere along the spectrum from drowsiness to being fully asleep, and is marked most easily by the appearance of drowsiness (decreased eye blinks, slow roving eye movements as discussed in the awake section), then a gradual loss of the PDR with coinciding diffuse attenuation of the tracing, and finally the presence of positive occipital sharp transients of sleep (POSTS) and vertex waves.

POSTS are a very descriptive waveform, as they are positive deflections seen in the occipital leads that look like sharps, but are not epileptiform, as seen below. Classically, they have a "sail-like" appearance, and can come in singles or runs. They arise in stage I sleep, but can persist throughout the later stages of sleep. POSTS have a cousin in the awake state called lambda waves, which are discussed in the artifact section.

Also found in stage I sleep are vertex waves, which are bilateral phase reversing discharges over the central regions. They can come alone or in runs of varying amplitude and morphology, but even when they appear very sharp or high amplitude they are not epileptiform. In children, vertex waves can be scary looking but still remain normal, as discussed in the pediatric section.

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This patient is awake but very drowsy. Recall that drowsiness is marked by diffuse attenuation and possibly mild slowing of the background, but you can still see a clear PDR in the posterior leads here. In the frontal and frontopolar regions, opposing slow undulations are seen in polarity, indicative of lateral roving eye movements. This occurs because the cornea is positively charged, and thus when you look to the right, the right eye's cornea gets closer to F8 and it sees a positive charge; at the same time, the left cornea moves away from F7 and thus it sees a negative charge. So, lateral eye movements lead to a frontal positive charge on the side to which you're looking, and a negative charge on the opposite side.

stage ii sleep

Stage II sleep is characterized by the arrival of sleep spindles and K complexes, although both vertex waves and POSTS persist into stage II from stage I sleep.

Sleep spindles are symmetric bursts of 12-14 Hz activity arising from the reticular nucleus of the thalamus; they are so named because they can look like spindles of thread, as seen below. The absence of spindles, or an asymmetry of their formation or frequency, should be considered abnormal.

K complexes
are very high amplitude, symmetric waveforms with initial negative followed by a slow positive phase, often but not always followed by a sleep spindle. They are named K complexes because in early sleep studies researchers noted that knocks or other noises while a patient was asleep brought them on.

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Stage II Sleep

Stage II sleep is characterized by all of the architecture from stage I -- including vertex waves and POSTS -- along with the arrival of sleep spindles and K complexes. Note the prevalence of POSTS on the first half of this page, then the high amplitude, diffuse and biphasic K complex followed quickly by a 12-14 Hz diffuse sleep spindle. This is an excellent example of a normal sleep tracing. There may also be a poorly formed vertex wave seen in the parasagittal leads in the fourth second of the page.

slow wave sleep

Slow wave sleep is marked by high amplitude, synchronized delta activity, usually 0.5-2 Hz in frequency. All of the architecture from earlier sleep phases (vertex waves, POSTS, spindles and K complexes) can be present in slow wave sleep, but usually become somewhat less organized, and often disappear altogether. Slow wave sleep can appear to be extremely high voltage, particularly in younger patients. Of note, medications such as benzodiazepines and barbiturates reduce the amount of this stages of sleep.

rapid eye movement sleep

Rapid eye movement, or REM sleep, increases in frequency across the span of the night. It is marked by a diffuse attenuation of amplitudes, with a range of frequencies amongst the background that can look similar to an awake state. It derives its name from the rapid eye movements that arise and, on EEG, are seen as very sharply contoured, opposing waveforms (with the upslope usually faster than the downslope) in the left and right frontal regions.

These waves arise because the cornea is positively charged, so when a patient looks to their right, the right eye cornea moves closer to the F8 electrode, leading to a positive charge seen at F8; at the same time on the left side, the left eye's cornea moves away from the F7 electrode, leading to a negative charge seen at F7. Thus, opposing frontal waveforms are seen: a positive wave on the side to which you're looking, and a negative wave on the opposite side.

In REM sleep one can also see some mild cardiac irregularity and breathing fluctuations, and muscle activity should be absent throughout this stage.undefined

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Stage II Sleep

This page shows several clear vertex waves, marked by high amplitude and symmetric discharges in the parasagittal regions and centrally, with a few less well formed ones in between the ones marked in blue boxes. There are also poorly formed POSTS in the occipital regions bilaterally. This tracing appears to be in the initial transition from stage I to stage II sleep, as some of the higher amplitude background activity in the middle and towards the end of the page are K complexes. Of note, the frontal bursts of very high frequency activity marked in green are lateral rectus muscle artifact (which are more classically seen in REM sleep); these are discussed in the artifact section.

Sleep is a series of four phases: stage I, stage II, slow wave sleep, rapid eye movement (REM)

Stage I sleep is marked by vertex waves and positive occipital sharp transients of sleep (POSTS)

Stage II sleep is marked by sleep spindles and K complexes

Slow wave sleep is marked by diffuse, synchronized delta activity

REM sleep has diffuse attenuation of activity with lateral eye movements in the frontal leads

REM Sleep

This tracing shows multiple eye movements, mostly lateral eye movements, and very little other activity. This diffuse attenuation of activity with the present of eye movements is classic for rapid eye movement (REM) sleep. Recall the cornea is positively charged and the retina is negative, so when the eyes look up it causes the retina to move down and the frontal electrodes see a positive charge (and thus a negative charge when the eyes look down). Similarly, when the eyes look to the right, the cornea moves laterally and closer to the right frontal electrode (F8), which sees a positive charge, while the left side sees a negative charge as that cornea moves medially and away from the left frontal electrode (F7, in this example).

Slow Wave Sleep

This tracing shows diffuse, high amplitude delta activity with very little other sleep architecture, consistent with slow wave sleep. Note that poorly formed vertex waves are seen in the beginning of the page, which is normal given that sleep architecture from prior stages usually persists into slow wave sleep, but becomes less apparent and less organized.

Of note, this tracing also shows a BET -- a benign epileptiform transient of sleep. Despite the name, this is a normal finding in sleep and is discussed more in the normal variants section. In brief, though, BETs differ from actual epileptiform discharges in that they are very small (

Stage II Sleep

This tracing is somewhat different than those you've seen already, mostly due to its use an unusual montage. This is a specialized montage with a focus on the temporal regions (the two chains on top), a transverse section that compares the central region left to right (the middle chain), and the parasagittal chains on the bottom. In this montage, its important to note that the most posterior (occipital) leads in the temporal chains are not at the bottom of those chains, but rather in the middle; the bottom two lines in the temporal chains are actually the lateral temporal leads. Understanding this, the occipital leads show some POSTS, while spindles are seen elsewhere, making this consistent with stage II sleep.

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stage i sleep

stage ii sleep

slow wave sleep

rem sleep