Hockenbury Discovering Psychology 5th txtbk

Biological and Environmental “Clocks” That Regulate Consciousness139The Suprachiasmatic NucleusThe Body’s ClockYour many circadian rhythms are controlled by a master biologicalclock—a tiny cluster of neurons in the hypothalamusin the brain. As shown in Figure 4.1, this cluster ofneurons is called the suprachiasmatic nucleus, abbreviatedSCN. The SCN is the internal pacemaker that governsthe timing of circadian rhythms, including thesleep–wake cycle (R. Moore, 2007).Keeping the circadian rhythms synchronized with oneanother and on a 24-hour schedule also involves environmentaltime cues. The most important of these cues isbright light, especially sunlight. In people, light detectedby special photoreceptors in the eye is communicated viathe visual system to the SCN in the hypothalamus (Berson& others, 2002; Drouyer & others, 2007).Pineal glandFirst thingin the morningMelatoninShortly afterlight exposureLater in the dayHow does sunlight help regulate the sleep–wake cycle and other circadianrhythms? As the sun sets each day, the decrease in available light is detected by theSCN through its connections with the visual system. In turn, the SCN triggers anincrease in the production of a hormone called melatonin. Melatonin is manufacturedby the pineal gland, an endocrine gland located in the brain.Increased blood levels of melatonin help make you sleepy and reduce activity levels.At night, blood levels of melatonin rise, peaking between 1:00 and 3:00 A.M.Shortly before sunrise, the pineal gland all but stops producing melatonin, and yousoon wake up. As the sun rises, exposure to sunlight and other bright light suppressesmelatonin levels, and they remain very low throughout the day. In this way,sunlight regulates, or entrains, the SCN so that it keeps your circadian cycles synchronizedand operating on a 24-hour schedule.Circadian Rhythms and Sunlight:The 24.2-hour DaySunlight plays a critical role in regulating yourinternal clock. So what would happen if youwere deprived of all environmental time cues,like sunlight/darkness cues, clocks, and schedules?In the absence of all external time cues, researchershave found that our internal bodyclock drifts to its natural—or intrinsic—rhythm. Interestingly, our intrinsic circadianrhythm is about 24.2 hours, or slightly longerthan a day (Czeisler & others, 1999). And ournormally coordinated circadian rhythms becomedesynchronized (Dijk & Lockley, 2002).For example, your sleep–wake, body temperature,and melatonin cycles are usually veryclosely coordinated. At about 3:00 A.M., yourbody temperature dips to its lowest point just asmelatonin is reaching its highest level and youare at your sleepiest. But when deprived of allenvironmental time cues, the sleep–wake, bodytemperature, and melatonin circadian rhythmsbecome desynchronized so that they are nolonger properly coordinated with one another.Pineal glandSuprachiasmaticnucleusOptic nerveFigure 4.1 The Biological Clock Specialphotoreceptors in the retina regulate theeffects of light on the body’s circadianrhythms (Menaker, 2003). In response tomorning light, signals from these specialphotoreceptors are relayed via the opticnerve to the suprachiasmatic nucleus. Inturn, the suprachiasmatic nucleus reducesthe pineal gland’s production of melatonin,a hormone that causes sleepiness.As blood levels of melatonin decrease,mental alertness increases. Daily exposureto bright light, especially sunlight, helpskeep the body’s circadian rhythms synchronizedand operating on a 24-hour schedule.Circadian Rhythms and the Blind Manyblind people have desynchronized circadianrhythms because they’re unable todetect the sunlight that normally sets thebody’s internal biological clock, the SCN.Like sighted people deprived of all environmentaltime cues, blind people canexperience melatonin, body temperature,and sleep–wake circadian cycles that operateindependently. Consequently, about 60percent of blind people suffer fromrecurring bouts of insomnia and othersleep problems (Arendt & others, 2005;Mistlberger & Skene, 2005).