EXCERPT
With this knowledge in hand, the AASM manual lays out specific rules for determining each stage.
- Stage W – The EEG contains alpha activity in the occipital region for more than half the epoch; or the epoch contains eye blinks, reading eye movements and/or irregular conjugate rapid eye movements associated with relatively high EMG tone.
- Stage N1 – Alpha rhythm is replaced by relatively low voltage, mixed frequency EEG activity for more than half the epoch; or theta activity (4-7 Hz) is predominate along with generalized slowing and/or vertex sharp waves and/or slow eye movements.
- Stage N2 – Sleep spindles and/or K-complexes (not associated with an EEG arousal) appear.
- Stage N3 – 20% of the epoch (or greater) consists of high amplitude slow waves in the EEG.
- Stage R – A relatively low voltage, mixed frequency EEG in conjunction with rapid eye movements and low amplitude EMG predominates.
In general, as sleep progresses from stages 1 through 4, the EEG increases in amplitude (gets larger) and decreases in frequency (gets slower).
Wakefulness actually has two distinct patterns. When the subject is awake and alert, the pattern is one of a relatively fast, mixed frequency, low voltage EEG. Although this pattern slows as the subject falls into deeper sleep, a background of mixed frequency, low voltage EEG is almost always visible in all stages. The other EEG pattern of wake is one of relaxation, usually with eyes closed. This consists of almost continual alpha waves, in the frequency of 8 – 13 hertz. Note that the alpha pattern will always be slower than the sleep spindles in the same subject.
The waves in stage N1 will be primarily in the theta range of 2 – 7 hertz. Vertex sharp waves may also be seen, occasionally as high as 200 µV in amplitude. Slow, rolling eye movements and reduced EMG amplitude are commonly seen in this stage, but these are not criteria for scoring.
Sleep spindles, which help determine stage N2, are defined as 12 – 14 hertz waves lasting at least 0.5 seconds. K-complexes are well-delineated negative sharp waves followed immediately by a positive component, and lasting at least 0.5 seconds. They may or may not occur in conjunction with spindles. There are no amplitude criteria for sleep spindles or K-complexes.
The slow waves of stage N3 are in the delta range of 0.5 to 2 hertz, and must be at least 75 µV in amplitude, peak-to-peak in the frontal regions. (Note: As we age, EEG amplitude often decreases. It is for this reason some laboratories ignore the voltage criterion for slow wave sleep and score slow waves by frequency alone.)
In REM sleep, the EEG is similar to stage N1, except no vertex waves will be seen, and saw tooth shaped waves in the theta band are often seen instead. Rapid eye movements must also occur, and the EMG of the chin must be lower in amplitude than in any other stage.
