2.2 The EEG and Sleep Architecture

The most commonly referenced EEG rhythms (frequency bands) are listed in Table 2.2.1 [Rechtschaffen and Kales, 1968; Clenney and Johnson, 1983].

Frequency Band	Range (Hz)
Delta	0.5-2
Theta	3-7
Alpha	8-12
Beta	13-30

Table 2.2.1 EEG Standardized Frequency Bands

Sleep is regarded as an active and complex state, and the term "sleep architecture" describes the stages and cycles of sleep.  There are two primary types of sleep: non-rapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep.  Within NREM sleep, three sleep stages are identified: stage 1, stage 2, and delta.  We will briefly review the NREM stages of sleep and because the first REM stages usually occur 70 to 90 minutes after sleep onset and this is well past the detection window of interest for alertness monitoring, REM sleep will not be discussed.  Frequency content in the EEG for sleep scoring is taken from Central (C) locations, although many sleep laboratories also use an Occipital (O) electrode which usually reveals higher alpha spectral energy for improved determination of the sleep onset period.  Much of the following summary information is from Orr and Hauri, 1982.

Stage 1 sleep is the transitional phase between wakefulness and sleep with many researchers identifying this stage as the sleep onset period (SOP).  This stage is characterized by dominant energy in the EEG signal in the theta band [Rechtschaffen and Kales, 1968].  This stage usually lasts from 30 seconds to several minutes in normal sleepers.  During this stage, reactivity to outside stimulus is diminished, thoughts begin to drift, and short dreams often develop.  If awakened during this stage, many people will report being awake and hence this stage is also called the "denial stage" of sleep.

Stage 2 sleep is identified by the introduction of K-complexes (high-voltage Vertex waves) and sleep spindles (bursts of 12-14hz activity) into the EEG.  In this stage, K-complexes can be observed in sleep as a response to auditory stimulation [Rechtschaffen and Kales, 1968; Clenney and Johnson, 1983].

Delta sleep (also called slow wave sleep) is dominated by energy in the delta band.  This is the deepest of the NREM sleep stages.  This stage is usually reached within 30 to 45 minutes from sleep onset.

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