Sleep is not a passive event, but rather it is an active process involving characteristic physiological changes in the organs of the body. Scientists study sleep by measuring the electrical changes in the brain using a technique called electroencephalography. This technique produces data in the form of electroencephalograms (E E G). Normally, electrodes are placed on the scalp; these are usually fairly numerous and placed in a symmetrical pattern. They measure very small voltages that are thought to be caused by synchronized activity in very large numbers of synapses (nerve connections) in the cerebral cortex. E E G data are represented by curves which are classified according to “rhythm.” The wavy lines of the E E G are what most people know as “brain waves.” Wakefulness and R E M stage sleep are both characterized by low amplitude, random, fast wave patterns. In contrast, NREM stage sleep is characterized by high amplitude, slow waves.
Scientists measure the electrical activity associated with active muscles, using electromyograms (E M G). This is accomplished by placing electrodes on the skin overlying a muscle. In humans, E M G is generally recorded by placing electrodes under the chin, since muscles in this area demonstrate very dramatic changes during the various stages of sleep. Electrodes may also be placed on the lower leg. During wakefulness, the E M G may vary between moderate and high, depending on the activities in which the individual is engaged. E M Gs in NREM stage sleep are moderate to low. In R E M stage sleep, voluntary muscle activity is inhibited and the E M G is virtually absent.
If an electrode is placed on the skin near the eye, changes in voltage are measurable as the eye rotates in its socket. This produces an electrooculogram (E O G). During wakefulness, rapid eye movements may be very frequent or scarce, depending on the extent to which vision is being used. Eye movement is absent during NREM, although some brain activity may be picked up by the testing equipment and be recorded incorrectly as eye activity. During R E M stage sleep, there are bursts of rapid eye movements, in between which there are periods of no eye movements.
Sleep is a highly organized sequence of events that follow a regular cycle each night. For instance, the E E G, E M G, and E O G patterns change in predictable ways several times during a single sleep period. Study of these events has led to the identification of two basic stages, or states, of sleep: non-rapid eye movement (NREM) sleep and rapid eye movement (R E M) sleep. Physiologic events, such as body temperature, blood pressure, heart rate, respiration, and hormone release are also different during wakefulness, NREM sleep, and R E M sleep. NREM sleep, also known as slow wave (SW) sleep, is subdivided into four stages according to the amplitude and frequency of brain wave activity, eye movement, and voluntary muscle activity that typify each substage. Generally, these four stages differ primarily in their E E G patterns, while the general physiology of these stages is fairly similar. Therefore, in this manual, emphasis will be on NREM sleep in general and not on its individual substages. Sleep is a cyclical process. During sleep, people experience repeated cycles of NREM and R E M sleep, beginning with an NREM phase. This cycle lasts approximately 90-110 minutes and is repeated three to six times per night. As the night progresses, however, the amount of NREM sleep decreases and the amount of R E M sleep increases. The figure graphically depicts the pattern of cycling we experience. The term ultradian rhythm (i.e., rhythm occurring with a frequency of less than 24 hours) is used to describe this cycling through sleep stages.
Hypnograms were developed to summarize the large amount of chart recordings (E E G, E M G, & E O G) that are made when recording electrical activities occurring during a night’s sleep. As a simple graphic, they provide a convenient means of evaluating data that would originally have been collected on many feet of chart paper or stored as a large digital file on a computer. This hypnogram summarizes how a typical night’s sleep for a young, healthy adult is organized.
Body temperature is relatively constant during wakefulness. However, it is maintained at a lower set point during NREM stage sleep, thus resulting in a lower body temperature during NREM as compared to wakefulness. Body temperature is not regulated during R E M stage sleep, and it will drift toward the environmental temperature. There is also a biological clock-related component to body temperature. This means that the body temperature will vary in a regular way with the time of day. For instance, body temperatures will be higher at mid-afternoon and reach their low point in the early morning hours before awakening.
During wakefulness, respiration may vary with activity, stress, and emotional levels. During NREM stage sleep, breathing slows, and the inhalation and exhalation of air decrease in magnitude as compared to wakefulness. Breathing during NREM sleep is generally very regular. In R E M stage sleep, breathing can be very irregular.
During wakefulness, heart rate (in bpm or beats per minute) can vary considerably depending on the level of activity in which the individual is engaged. During NREM stage sleep, the heart rate exhibits less variability and may be slightly lower than what is observed during resting or less active wakefulness. Heart rate during R E M stage sleep exhibits pronounced changes and may rise to levels seen during moderate to strenuous exercise.
During wakefulness, blood pressure can vary considerably, for instance, with activity and stress levels. Blood pressure tends to decrease slightly during NREM stage sleep and exhibit less variability. During R E M stage sleep, blood pressure is highly variable and may occasionally increase up to 30% over the resting level. During R E M sleep, the diameter of blood vessels decreases (i.e., they undergo vasoconstriction), which may be the cause of the rise in blood pressure.
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