The stages of sleep usually follow
an orderly, predictable sequence. These stages are
documented in sleep laboratories using a polygraph.
This device records the electrical activity of the
brain through small metal discs attached to the scalp.
The recordings produce an electroencephalogram (EEG).
Electrical signals from other parts of the body, for
example the heart, eye movements or muscles, or even
various physiological states such as respiration or
temperature, can also be recorded to aid sleep studies.
When a person is alert, the EEG shows irregular tracings
of high-frequency, low-amplitude waves known as beta
activity. Alpha activity, or medium-frequency, medium-amplitude
rhythm on the other hand, occurs when the subject
is in a relaxed or drowsy state.
The first stage of sleep is actually
a transition period between wakefulness and sleep.
Characterised by theta activity and muscle movements,
especially in the area of the eyes.
Foreword
