Hockenbury Discovering Psychology 5th txtbk

Sleep143It can easily take 15 minutes or longer to regain full waking consciousness fromstage 4. It’s even possible to answer your cell phone or respond to a text message,carry on a conversation for several minutes, and hang up without ever leaving stage4 sleep—and without remembering having done so the next day. When people arebriefly awakened by sleep researchers during stage 4 NREM and asked to performsome simple task, they often don’t remember it the next morning.Thus far, the sleeper is approximately 70 minutes into a typical night’s sleep andimmersed in deeply relaxed stage 4 NREM sleep. At this point, the sequence reverses.In a matter of minutes, the sleeper cycles back from stage 4 to stage 3 tostage 2 and enters a dramatic new phase: the night’s first episode of REM sleep.sleep spindlesShort bursts of brain activity that characterizestage 2 NREM sleep.K complexSingle but large high-voltage spike of brainactivity that characterizes stage 2 NREMsleep.REM SleepDuring REM sleep, the brain becomes more active, generating smaller and fasterbrain waves. Visual and motor neurons in the brain activate repeatedly, just as theydo during wakefulness. Dreams usually occur during REM sleep. Although thebrain is very active, voluntary muscle activity is suppressed, which prevents thedreaming sleeper from acting out his or herdreams.REM sleep is accompanied by considerablephysiological arousal. The sleeper’s eyesdart back and forth behind closed eyelids—the rapid eye movements. Heart rate, bloodpressure, and respirations can fluctuate upand down, sometimes extremely. Muscletwitches occur. In both sexes, sexual arousalmay occur, which is not necessarily related todream content.This first REM episode tends to be brief,about 5 to 15 minutes. From the beginning ofstage 1 NREM sleep through the completion ofthe first episode of REM sleep, about 90 minuteshave elapsed.Beyond the First 90 MinutesThroughout the rest of the night, the sleeper cycles between NREM and REMsleep. Each sleep cycle lasts about 90 minutes on average, but the duration of cyclesmay vary from 70 to 120 minutes. Usually, four more 90-minute cycles of NREMand REM sleep occur during the night. Just before and after REM periods, thesleeper typically shifts position.The progression of a typical night’s sleep cyclesis depicted in Figure 4.3. Stages 3 and 4 NREM,slow-wave sleep, usually occur only during thefirst two 90-minute cycles. As the night progresses,REM sleep episodes become increasinglylonger and less time is spent in NREM. Duringthe last two 90-minute sleep cycles before awakening,NREM sleep is composed primarily ofstage 2 sleep and periods of REM sleep that canlast as long as 40 minutes. In a later section, we’lllook at dreaming and REM sleep in more detail.Changing Sleep Patterns over the LifespanThe different elements of sleep first emerge during prenatal development. Duringthe last trimester of prenatal development, active (REM) and quiet (NREM) sleepcycles emerge. In the final weeks, REM and NREM sleep are clearly distinguishablein the fetus (Mirmiran & others, 2003).