第4章意識の状態

Sleep is not a uniform state of being. Instead, sleep is composed of several different stages that can be differentiated from one another by the patterns of brain wave activity that occur during each stage. While awake, our brain wave activity is dominated by beta waves. As compared to the brain wave patterns while asleep, beta waves have the highest frequency (13–30 Hz) and lowest amplitude, and they tend to show more variability. As we begin to fall asleep, our brain wave activity changes. These changes can be visualized using an EEG and are distinguished from one another by both the frequency and amplitude of the brain wave. The frequency of a brain wave is how many brain waves occur in a second, and frequency is measured in Hertz (Hz). Amplitude is the height of the brain wave (Figure 4.7). Sleep can be divided into two different general phases: REM sleep and non-REM (NREM) sleep. Rapid eye movement (REM) sleep is characterized by darting movements of the eyes under closed eyelids. Brain waves during REM sleep appear very similar to brain waves during wakefulness. In contrast, non-REM (NREM) sleep is subdivided into three stages distinguished from each other and from wakefulness by characteristic patterns of brain waves. The first three stages of sleep are NREM sleep, typically followed by REM sleep. In this section, we will discuss each of these stages of sleep and their associated patterns of brain wave activity.

As we begin to fall asleep, we enter NREM sleep, and brain wave patterns decrease in frequency and increase in amplitude. The first stage of NREM sleep is known as stage 1 sleep. Stage 1 sleep is a transitional phase that occurs between wakefulness and sleep, the period during which we drift off to sleep. During this time, there is a slowdown in both the rates of respiration and heartbeat. In addition, stage 1 sleep involves a marked decrease in both overall muscle tension and core body temperature.

In terms of brain wave activity, stage 1 sleep is associated with both alpha and theta waves. The early portion of stage 1 sleep produces alpha waves. These patterns of electrical activity (waves) resemble that of someone who is very relaxed, yet awake, but they have less variability (are more synchronized) and are relatively lower in frequency (8–12 Hz) and higher in amplitude than beta waves (Figure 4.8). As an individual continues through stage 1 sleep, there is an increase in theta wave activity. Theta waves are even lower frequency (4–7 Hz), and higher in amplitude, than the alpha wave patterns. It is relatively easy to wake someone from stage 1 sleep; in fact, people often report that they have not been asleep if they are awoken during stage 1 sleep.

As we move into stage 2 sleep, the body goes into a state of deep relaxation. Theta waves still dominate the activity of the brain, but they are interrupted by brief bursts of activity known as sleep spindles (Figure 4.9). A sleep spindle is a rapid burst of higher frequency brain waves that may be important for learning and memory (Fogel & Smith, 2011; Poe, Walsh, & Bjorness, 2010). In addition, the appearance of K-complexes is often associated with stage 2 sleep. A K-complex is a very high amplitude pattern of brain activity that may in some cases occur in response to environmental stimuli. Thus, K-complexes might serve as a bridge to higher levels of arousal in response to what is going on in our environments (Halász, 1993; Steriade & Amzica, 1998).

NREM stage 3 sleep is often referred to as deep sleep or slow-wave sleep because this stage is characterized by low frequency (less than 3 Hz), high amplitude delta waves (Figure 4.10). These delta waves have the lowest frequency and highest amplitude of our sleeping brain wave patterns. During this time, an individual’s heart rate and respiration slow dramatically, and it is much more difficult to awaken someone from sleep during stage 3 than during earlier stages. Interestingly, individuals who have increased levels of alpha brain wave activity (more often associated with wakefulness and transition into stage 1 sleep) during stage 3 often report that they do not feel refreshed upon waking, regardless of how long they slept (Stone, Taylor, McCrae, Kalsekar, & Lichstein, 2008).