Hockenbury Discovering Psychology 5th txtbk

138 CHAPTER 4 Consciousness and Its VariationsBiological and Environmental “Clocks”That Regulate ConsciousnessKey Theme• Many body functions, including mental alertness, are regulated by circadianrhythms, which systematically vary over a 24-hour period.Key Questions• How do sunlight, the suprachiasmatic nucleus, and melatonin regulate thesleep–wake cycle?• How do free-running conditions affect circadian rhythms?• What is jet lag, and how is it produced?The Emergence of Circadian RhythmsConsistent daily variations in movement,heart rate, and other variables are evidentduring the fifth month of gestation in thehuman fetus. After birth, the synchronizationof infants’ circadian rhythms to aday–night cycle usually occurs by 2 or 3months of age (Mistlberger & Rusak, 2005).Daytime exposure to bright light helps establishthese regular rest–activity circadianrhythms.Throughout the course of each day, there is a natural ebb and flow to consciousness.The most obvious variation of consciousness that we experience is the dailysleep–wake cycle. However, researchers have identified more than 100 other physicaland psychological processes that rhythmically peak and dip at consistent timeseach day, including blood pressure, the secretion of different hormones, mentalalertness, and pain sensitivity.Each of those examples represents a specific circadian rhythm. The word circadiancombines the Latin words for “about” and “day.” So, the term circadianrhythm refers to a biological orpsychological process that systematicallyvaries over the courseof each day.Normally, your different circadianrhythms are closely synchronized.For example, the circadianrhythm for the release of growthhormone is synchronized withthe sleep–wake circadian rhythmso that growth hormone is releasedonly during sleep. Table4.1 lists other examples of circadianrhythms.Table 4.1Examples of Human Circadian Rhythmscircadian rhythm(ser-KADE-ee-en) A cycle or rhythm that isroughly 24 hours long; the cyclical dailyfluctuations in biological and psychologicalprocesses.suprachiasmatic nucleus (SCN)(soup-rah-kye-az-MAT-ick) A cluster of neuronsin the hypothalamus in the brain thatgoverns the timing of circadian rhythms.melatonin(mel-ah-TONE-in) A hormone manufacturedby the pineal gland that produces sleepiness.FunctionTypical Circadian RhythmPeak mental alertness and memory Two daily peaks: around 9:00 A.M. and 9:00 P.M.Lowest body temperatureAbout 97°F around 4:00 A.M.Highest body temperatureAbout 99°F around 4:00 P.M.Peak hearing, visual, taste, and smell Two daily peaks: around 3:00 A.M. and 6:00 P.M.sensitivityLowest sensitivity to painAround 4:00 P.M.Peak sensitivity to painAround 4:00 A.M.Peak degree of sleepinessTwo daily peaks: around 3:00 A.M. and 3:00 P.M.Peak melatonin hormone in blood Between 1:00 A.M. and 3:00 A.M.Sources: Campbell (1997); Czeisler & Dijk (2001); Refinetti (2000); M. Young (2000).