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Early in sleep-related
research, standardized criteria were developed for scoring the
data from each of the somnographic channels and they became the
"gold standard" for describing physiological sleep (Rechtschaffen
et al., 1968). These scoring standards facilitated reliable
sleep-stage scoring within and between laboratories and established
standard terminology for communicating findings across investigators.
Using this
standard scoring protocol, descriptive studies showed that normative
human sleep incorporates consistent cyclic patterns of PSG changes,
expressed as sleep stages. These include four stages of non-rapid
eye movement (non-REM) sleep and one stage of rapid eye movement
(REM) sleep, the latter consisting of very obvious, rapid saccadic
eye movements. Other physiologic status indicators vary across
sleep stages, and sleep-related disorders such as breathing apnea
and periodic leg movements can disturb sleep. Therefore, PSG monitoring
frequently includes monitoring of heart rate, breathing, leg movements,
and sometimes blood pressure and body temperature. Alternatives
to PSG for measuring sleep and waking are behavioral observations
and activity monitors placed on the wrist or ankle. These alternatives,
however, cannot provide much detail in sleep structure.
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