Hockenbury Discovering Psychology 5th txtbk

142 CHAPTER 4 Consciousness and Its Variations50 to 70 minutes5 to 15 minutesAwakeand Alert:Beta brain wavesAwakebut Drowsy:Alpha brainwavesStage 1NREM Sleep:Mixture ofalpha and thetabrain wavesSleepspindlesK complexStage 2NREM Sleep:Sleep spindles,K complexes,theta brain waves,and beginningsof delta wavesStage 3NREM Sleep:Mixture oftheta and deltabrain wavesStage 4NREM Sleep:Delta brain wavesREM Sleep:Fast, activebrain wavesaccompaniedby rapid eyemovements(REMs)Figure 4.2 The First 90 Minutes ofSleep From wakefulness to the deepestsleep of stage 4 NREM, the brain’s activity,measured by EEG recordings, progressivelydiminishes, as demonstrated by larger andslower brain waves. The four NREM stagesoccupy the first 50 to 70 minutes of sleep.Then, in a matter of minutes, the braincycles back to smaller, faster brain waves,and the sleeper experiences the night’sfirst episode of dreaming REM sleep,which lasts 5 to 15 minutes. During therest of the night, the sleeper continues toexperience 90-minute cycles of alternatingNREM and REM sleep.Source: Based on Carskadon & Dement (2005).Synchronized Sleepers As these time-lapsephotographs show, couples who regularlysleep in the same bed tend to have synchronizedsleep cycles. Since bed partners fallasleep at about the same time, they arelikely to have similarly timed NREM–REMsleep cycles. The movements of this coupleare also synchronized. Both sleepers shiftposition just before and after episodes ofREM sleep.Stage 2 NREMStage 2 represents the onset of true sleep. Stage 2 sleep is defined by the appearance ofsleep spindles, brief bursts of brain activity that last a second or two, and K complexes,single high-voltage spikes of brain activity (see Figure 4.2). Other than these occasionalsleep spindles and K complexes, brain activity continues to slow down considerably.Breathing becomes rhythmical. Slight muscle twitches may occur. Theta wavesare predominant in stage 2, but larger, slower brain waves, called delta brain waves,also begin to emerge. During the 15 to 20 minutes initially spent in stage 2, deltabrain-wave activity gradually increases.Stage 3 and Stage 4 NREMIn combination, stages 3 and 4 are often referred to as slow-wave sleep (SWS). Bothstages are defined by the amount of delta brain-wave activity. When delta brainwaves represent more than 20 percent of total brain activity, the sleeper is said to bein stage 3 NREM. When delta brain waves exceed 50 percent of total brain activity,the sleeper is said to be in stage 4 NREM.During the 20 to 40 minutes spent in the night’s first episode of stage 4 NREM,delta waves eventually come to represent 100 percent of brain activity. At that point,heart rate, blood pressure, and breathing rate drop to their lowest levels. Not surprisingly,the sleeper is almost completely oblivious to the world. Noises as loud as90 decibels may fail to wake him. However, his muscles are still capable of movement.For example, if sleepwalking occurs, it typically happens during stage 4NREM sleep.