To decide if someone is sleeping or lying awake with eyes closed, we need a brain current diagram. The German psychiatrist Hans Berger was the first person to measure electrical activity of the cerebral cortex by the use of electrodes attached to the scalp. He called this method "Elektroenzephalography" (EEG), creating the basis for modern sleep medicine: soon, the difference between the EEG of waking and sleeping subjects was noticed. It was observed that in sleep, the amplitude of the brain waves were higher than those the EEG registered with persons who were awake.
In a modern sleep facility, parameters of sleeping persons are measured continuously throughout the night. It is necessary for the classification of sleep into different stages to record the so-called elektrooculogram (EOG) and electromyogram (EMG) additionally to the EEG.
The EOG registers the movement of the eyeballs. This method uses the fact that - unlike the cornea - the retina is charged negatively. By use of electrodes attached in the proximity of the eyeballs, electrical potentials can be registered when the eyes move.
During the process of falling asleep, a slow, rolling movement of the eyes can be observed. The so-called REM-sleep (rapid eye movements) was named after fast, conjugated movements of the eyes, which usually can be observed only while awake. As the EEGs of the REM sleep and the waking condition are rather similar, we need another parameter to differentiate between them:
The EMG is an indicator for tension of the muscular system. The tension of the muscles lessens, the deeper our sleep is. We reach the lowest point of muscle tension during REM sleep, whereas in the waking condition, we reach the maximum muscle tension. For the EMG recording, two electrodes are attached to the skin above the muscular system of the chin.
Using the three parameters EEG, EOG and EMG, it is possible to to divide sleep into three different stages. The factors relevant to the evaluation are frequency and amplitude of the EEG on one hand and the occurrence of graphoelements typical for the appropriate stages of sleep. We subdivide in wakefulness and sleep. The condition of sleep is subdivided again into REM sleep and NREM sleep (non-REM sleep). NREM sleep is subdivided into stages I-IV.
The rules of sleep classification have been put down by a commission of experts led by A.Rechtschaffen and A.Kales in 1968, so the results of different sleep facilities have been comparable since that time.
Fig. 1 illustrates the standard positioning of the electrodes (from: Rechtschaffen, Kales: A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects, Public Health Service, U.S. Government, Printing Office, Washington, D.C., 1968.).
Survey of the Stages of Sleep
We subdivide the night into periods of 30 secs. A trained evaluator then assigns each of the periods to one of the stages of sleep. The organization into periods of 30 seconds is arbitrary and is based on the fact that during the early days of sleep medicine, the EEGs were recorded on paper - and with a paper speed of 10mm/s, exactly 30 seconds fit on one page.
Based on this periodization, a night that lasts 8 hours contains 960 periods, which have to be assessed individually. For the nocturnal aircraft noise studies, this means that in approx. 2500 subject nights, approx. 2500 x 960 = 2.4 millon periods will have to be evaluated.
Following the allocation by sleep stages, modified by so-called "smoothing" criteria, we can provide a hypnogram that plots the temporal operational sequence of the sleep stage sequence in form of a line reminiscent of a stairs.
After falling asleep (first time sleep stage II), the healthy sleeper will usually quickly slide into sleep stage III and IV, which are summarily called deep sleep. The designation "deep sleep" is based on the fact that the possibiliy of being woken up by external stimuli in sleep stages III and IV is less high than in the so-called "light sleep" (stages I and II). We speak of a synchronisation of the EEG and the so-called slow wave sleep (SWS). After a period of deep sleep, the human sleep will usually abruptly become lighter and the REM sleep stage follows.
With the end of the first REM sleep stage, the first cycle of sleep is finished. One sleep cycle lasts approx.. 90 to 110 min. It will be repeated 3 to 6 times during the night. The duration of deep sleep is longer during the first half of the night than it is during the second half , whereas the duration of REM sleep is longer during the second half of the night than it is during the first one. Even with healthy persons, periods of waking arise occasionally. Short waking reactions taking the form of EEG accelerations are called arousal. These can be observed several times an hour with the healthy sleeper, as well.
While the parameters EEG, EOG and EMG described above are sufficient for a pure sleep stage analysis, further parameters have to be recorded continually and parallel to the measured variables mentioned above. In the clinical routine, those variables usually are:
Besides EEG, EOG and EMG, the following parameters are recorded in the context of the aircraft noise study STRAIN: (see section Physiological Variables):
Sleep has a regenerative function which is partially bound to the occurence of certain sleep stages. The occurence of deep sleep (NREM III and IV) has special importance.This fact is based on observances during sleep withdrawal tests among other things. After a period of sleep withdrawal, the test subjects mainly made good deep sleep and -less strongly- REM sleep. Selective deep sleep withdrawal also leads to severe performance impairments by day.
We also know that frequent waking reactions and even shorter EEG accelerations in the form of arousal may influence performance and wakefulness the following day.It will be the aim of the aircraft noise study STRAIN to find out the number and volume level of nocturnal aircraft noises neceassary to produce a recognizable unnatural increase of waking reactions and/or significant changes in the sleeping structure, and to find out from which point on these changes lead to an impairment of performance by day.
We should stress that a certain number of waking reactions and arousal are a natural phenomenon that is part of healthy sleep.
Further Reading on "Sleep" hereand Links on the Internet on "Sleep" here