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Chapter 11

Thorough Notes on Chapter 11

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University of Toronto Scarborough
Janelle Leboutillier

Physio Chapter 11 Sleep and Waking Circadian Rhythms Arepeating cycle of about 24 hours. Together, the interplay of sleep and waking cycles follow circadian, or daily, rhythms. To establish and maintain these rhythms, internal biological clocks interact with stimuli known as zeitgebers: an external cue for setting biological rhythms Light is the most important zeitgeber for human beings. In the absence of natural light, human free-running circadian rhythms (Arhythm that is not synchronized to environmental time cues) last approximately 24.2 hours to 24.9 hours Exposure to sunlight each day helps reset, or entrain, the internal biological clock to the 24- hour cycle of the earths rotation. Totally blind people and sailors on submarines experience free-running cycles that are longer than 24 hours, often resulting in severe sleep disruptions In addition to light, other zeitgebers include physical activity, feeding, body temperature, and sleep-related hormones discussed later in this chapter Variations in Sleep Patterns People who are most alert and productive in the morning have been referred to as larks, whereas night people have been referred to as night owls Regardless of normal lifetime sleep patterns, nearly everyone acts like an owl during adolescence because Melatonin, one of the neurochemicals involved in the regulation of sleep patterns, drops dramatically at the onset of puberty Following adolescence, many temporary owls will revert to their previous state, possibly due to the maturation of neural systems that regulate sleep. Regardless of the origins of adolescent owl behavior, accommodations can be useful. Ex: Shifting from a 7:15 a.m. start time to an 8:40 a.m. start time improved both attendance and student grades at Minnesota high schools (Wahlstrom, 2003). Shift Work, Jet Lag, and Daylight Saving Time When work demands and circadian rhythms do not match, the consequences can be challenging and possibly dangerous. Between 40 and 80 percent of workers on the 11 p.m. to 7:30 a.m. night shift experience disturbed sleep and a cluster of symptoms referred to as shift maladaptation syndrome These workers obtain 1.5 hours less total sleep than workers on other shifts, leading to frequent health, personality, mood, and interpersonal problems.Accidents are also highest during these over night shifts Night shift workers are more likely than other workers to develop breast cancer Shift workers are not only a risk to themselves, but their errors also jeopardize the public. Hospital workers, such as nurses, are much more likely to make significant errors during evening or night shifts than during day shifts Conflicts between internal clocks and external zeitgebers also result in the unsettling experience of jet lag: After crossing time zones, people often experience fatigue, irritability, and sleepiness Chronic jet lag might have more serious consequences.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 mistakes on memory tasks than local crews who did not cross time zones Internal Clocks The bodys internal master clock is the suprachiasmatic nucleus (SCN) in the hypothalamus The SCN is only active during the day. This is true regardless of whether a species is diurnal (awake during the day), like monkeys, or nocturnal (awake at night), like rats.\ The SCN helps animals distinguish between day and night, but other structures dictate whether an animal is nocturnal or diurnal in its behavior The SCN is not dependent on input from other structures to maintain its rhythms. Isolated SCN tissue cultures continued to show rhythmic fluctuations in activity consistent with the source animals previous daynight cycle Transplants of SCN tissue also support its role as a master internal clock It is possible to breed hamsters with short free-running cycles of about 20 hours, in comparison with the normal hamster cycle of about 24 hours When SCN tissue from a short-period hamster is transplanted into a normal hamster, the normal hamster shows the short free-running cycle. When SCN tissue from a normal hamster is transplanted into a short-period hamster, the hamster shows normal 24-hour cycles The SCN acts as a master clock that coordinates the activities of other internal, peripheral clocks that exist in most body cells. Cells from the SCN, liver, lung, and muscle of rats were observed following six-hour phase shifts in the ratslight-dark schedules The SCN adjusted to the new time after only one or two cycles of light and dark, but peripheral clocks in the other tissues were much slower to respond Lung and muscle tissue required 6 cycles to adjust to the new time, and the liver required more than 16 cycles. The effects of phase shifts on muscles, lungs, and other tissues appear to last long after the initial discomfort is gone 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 Many travelers attempt to compensate forjet lag by immediately adjusting their mealtimes to their current time zone The Cellular Basis of Circadian Rhythms three separate genes and their protein products that are involved with cellular circadian rhythms Tim, Per, and Clock Together, per and tim proteins inhibit the Clock protein, whereas the Clock protein promotes the production of more per and tim proteins. Consequently, as levels of per and tim proteins increase, inhibition of the Clock protein ensures that no further per and tim proteins will be produced. 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 proteins Neural activity reflects the oscillation of the levels of these internal proteins, providing a mechanism for communicating rhythms to other cells. Biochemistry and Circadian Rhythms The SCN both regulates and responds to the hormone melatonin, an indoleamine secreted by the pineal gland Lesions of the SCN abolish the circadian release of melatonin. Melatonin levels are very low during the day, begin to rise in the hours before sleep, and usually peak at about 4 a.m., a time when nearly everybody finds it very difficult to stay awake Totally blind individuals experience a melatonin peak at a different time each day, often leading to sleep difficulties. People with pineal gland tumors or other medical conditions affecting melatonin report sleep problems Melatonin release is suppressed by light;Although bright lights are more likely to suppress melatonin, dimmer lights typical of indoor lighting also have the ability to suppress production and release Melatonin supplements have been reported to improve cases of jet lag, shift maladaptation syndrome, and other sleep disorders SeasonalAffective Disorder During the winter months at higher latitudes (areas closer to the poles of the Earth), the reduction in daylight hours can interfere with circadian rhythms. Consequently, some people will experience a type of depression known as seasonal affective disorder (SAD). Serotonin levels typically drop in the fall and winter, and people vulnerable to SAD might experience a greater than normal decrease, leading to symptoms of depression SAD might also be influenced by disruptions in melatonin release caused by uneven patterns of daily light SAD is treated by exposure to bright lights, with or without melatonin and antidepressants. Light therapy administered at dawn corrects cases in which people stay up too late, whereas light therapy in the evening helps people who are sleepy too early Icelanders experience lower rates of SAD; might enjoy protective
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