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PSYB64H3 (197)
Chapter 11

Chapter 11

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University of Toronto Scarborough
Janelle Leboutillier

CHAPTER 11 SLEEP AND WAKINGCIRCADIAN RHYTHMS y Our focus in this chapter is on the rhythms associated with sleep and wakefulness o Together the interplay of sleep and waking cycles follow circadian or daily rhythms o Circadian rhythm a repeating cycle of about 24 hours y To establish and maintain these rhythms internal biological clocks interact with stimuli known as zeitgebersZeitgeber An external cue for setting biological rhythms o o Light is the most important zeitgeber for human beings o In the absence of natural light human freerunning circadian rhythms last approximately 242 hours to 249 hours o Free running circadian rhythm a rhythm that is not synchronized to environmental times cues y Exposure to sunlight each day helps reset or entrain the internal biological clock to the 24hour clock of the earths rotation Entrainment the resetting of internal biological clocks to the 24hour cycle of the earths rotationo y In addition to light other zeitgebers include physical activity feeding body temperature and sleeprelated hormones VARIATIONS IN SLEEP PATTERNS y Individual sleep patterns result from different versions of the genes responsible for our internal clocks y People who are most alert and productive in the mornings have been referred to as larks whereas night people have been referred to as night owls y Melatonin one of the neurochemicals involved in the regulation of sleep patterns drops dramatically at the onset of puberty possibly contributing to agerelated changes in sleep habitsy Following adolescence many temporary owls will revert to their previous state possible due the maturation of neural systems that regulate sleep y Regardless of the origins of adolescent owl behavior accommodations can be useful SHIFT WORK JET LAG AND DAYLIGHT SAVING TIME y When work demands and circadian rhythms do not the consequences can be challenging and possibly dangerous y Between 40 and 80 percent of workers on the 11 PM to 730 AM night shift experience disturbed sleep and a cluster of symptoms referred to as shift maladaptation syndromey These workers obtain 15 hours less total sleep than workers on other shifts leading to frequent health personality mood and interpersonal problemsy Night shift workers are more likely than other workers to develop breast cancer y Shift workers are not only a risk to themselves but their errors also jeopardize the public y Conflicts between internal clocks and external zeitgebers also result in the unsettling experience of jet lag o Jet lagfatigue irritability and sleepiness resulting from travel across time zones y Chronic jet lag might have more serious consequences o Airline flight attendants who crossed time zones at least once a week for four or more years had reduced reaction times and made 9 percent more mistakesy It is easier to adjust to phase delay of our cycle setting the clock to a later point than to a phaseadvance setting the clock to an earlier point o A New Yorker travelling to Los Angeles goes to bed three hours later but then has the opportunity to sleep later o A Los Angeles resident travelling to New York goes to bed three hours early and awakens in what feels like in the middle of the night y Daylight saving time offers another opportunity to observe our response to phase shiftso Daylight saving time requires the settings of clocks to forwards one hour in spring a phase advance and back one hour in fall a phase delayo The fall shift is equivalent to westward travel and produces relatively little disruption o The spring shift produces symptoms similar to jet lag for a day or twoy Coren correlated Canadian traffic accident data with daylight saving time shifts o The phasedelay shift in fall resulted in a 7 percent decrease in traffic accidents on the following Mondayo In the phaseadvance shift in spring Cohen observed a comparable 7 percent increase in traffic accidentsINTERNAL CLOCKS y The bodys internal master clock in the suprachiasmatic nucleus SCN in the hypothalamus CHAPTER 11 SLEEP AND WAKINGo Suprachiasmatic nucleus SCN an area of the hypothalamus located above the optic chiasm responsible for maintaining circadian rhythms y The term suprachiasmatic comes from the structures location above the optic chiasmo Axons of special retinal ganglion cells known as nonimage forming NIF cells leave the optic nerve and project to the SCN forming the retinohypothalamic pathwayo Retinohypothalamic pathway a pathway leading from the retina of the eye to the hypothalamus provides light information necessary for the maintenance of circadian rhythms o Unlike other retinal ganglion cells the NIF cells do not process information about visual imageso The NIF cells contain a photopigment known as melanopsin that is related to but different from the other photopigments involved in vision o Melanopsin a photo pigment used by nonimage forming NIF retinal cellso The SCN is active only during the dayThe SCN helps animals distinguish between day and night but other structures dictate whether an animal is nocturnal awake at night or diurnal awake during the day in its behavioro The SCN is not dependent on input from other structures to maintain its rhythms o Isolated SCN tissue cultures continued to show rhythmic fluctuations in activity consistent with the source animals previous daynight cycle o Transplants of SCN tissue also support its role as a master internal clock When SCN tissue from a shortperiod hamster is transplanted into a normal hamster the normal hamster shoes the short freerunning cycleWhen SCN tissue from a normal hamster is transplanted into a shortperiod hamster the hamster shows normal 24hour cycles y In both cases the behavior of the transplant recipient matches the behavior of the donor o The SCN acts as a master clock that coordinates the activities of other internal peripheral clocks that exist in most body cells o The rhythms of the SCN are heavily influenced by the presence of light In contrast the peripheral clocks are more easily influenced by daily feeding cycles o Many travelers attempt to compensate for jet lag by immediately adjusting their mealtimes to their current time zoneTHE CELLULAR BASIS OF CIRCADIAN RHYTHMS y How can a structure like the SCN tell time o The answer lies in the oscillation of protein production and degradation of within a celly The ebbing and flowing of special proteins require approximately 24 hours o These genes and their proteins are per for period tim for timeless and clock circadian locomotor output cycles kaput o Together per and tim proteins inhibit the Clock protein whereas the Clock protein promotes the production of more per and timo Consequently as levels of per and tim proteins increase inhibition of the Clock protein ensures that no further per and tim proteins will be producedo As levels of per and tim proteins drop over time the reduced inhibition of the Clock protein results in increased production of more per and tim proteinsy Neural activity reflects the oscillation of the levels of these internal proteins providing a mechanism for communication rhythms to other cells BIOCHEMISTRY AND CIRCADIAN RHYTHMSy The SCN both regulates and responds to the hormone melatonin an indoleamine secreted by the pineal glando Melatonin an indoleamine secreted by the pineal gland that participates in the regulation of circadian rhythmsy Lesions of the SCN abolish the circadian release of melatonin y Melatonin levels are very low during the day begin to rise in the hours before sleep and usually peak at about 4 am a time when nearly everybody finds it very difficult to stay awakey People with pineal gland tumors or other medical conditions affecting melatonin report sleep problemsy Melatonin release is suppressed by light y Although bright lights are more likely to suppress melatonin dimmer lights typical of indoor lighting also have the ability to suppress production and release
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