Dr. Troxel said the ritual of going to bed with a partner can trigger social "zeitgebers," or cues that influence circadian rhythms.
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For example, Jürgen Aschoff showed that individuals can compensate for the absence of some zeitgebers like natural light by attending to social zeitgebers instead. Specifically, individuals placed in total darkness for four days did not differ on a variety of measures, including body temperature and various psychomotor tasks like time estimation and finger tapping, from individuals placed in an artificial light-dark environment when both groups were given the same strict time schedule. Researchers concluded that social zeitgebers, like meal times and interactions with other people, can entrain biological rhythms in ways similar to those of other common zeitgebers like light.
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Since the internal clock sets itself using zeitgebers, the loss or disruption of an individual's usual zeitgebers can be very disorienting. When an individual experiences significant changes in zeitgebers, such as being irregularly scheduled for the night shift, those changes can have a variety of negative effects. One example of this phenomenon is jetlag, in which traveling to another time zone causes desynchronization in sleep-wake cycles, appetite, and emotions. Such zeitgeber disruptions can also lead to decreased cognitive performance, negative mood, and in some cases, episodes of mental illness.
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Zeitgebers (“time givers”) are environmental cues that synchronize biological rhythms to the 24-hour light/dark cycle. As the sun is a physical zeitgeber, social factors are considered social zeitgebers. These include personal relationships, social demands, or life tasks that entrain circadian rhythms. Disruptions in circadian rhythms can lead to somatic and cognitive symptoms, as seen in jet lag or during daylight saving time.
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These zeitgebers do so by alerting individuals to changes in the likelihood of possible rewards or threats in the environment. For example, humans are more likely to find food and shelter in the daytime and less likely to detect predators in the nighttime, meaning wakefulness tends to be most fruitful during the day and sleep is the safest activity for the nighttime. Therefore, changes in light and darkness influence the body to rise during the day and become fatigued at night. There are many different zeitgebers, and their relative influence on an individual at any given time depends on a number of factors, including the presence and operation of other kinds of zeitgebers.
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TPL requires spatial memory and a sense of time. The latter may be based on external time-cues (Zeitgebers), or internally generated circadian rhythms ("biological clock"). TPL may fundamentally underlie episodic memory.
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Disturbances in zeitgebers can exert a negative influence on emotion and mood as well as cognitive functioning. The disturbance of biological rhythms by zeitgebers is theorized to increase risk for some forms of psychopathology. There is strong evidence that individuals with depression experience irregular biological rhythms, including disrupted sleep-wake cycles, temperature, and cortisol rhythms. These findings support the theory first proposed by Ehlers, Frank, and Kupfer in 1988 that says that stressful life events can lead to depressive episodes by disrupting social and biological rhythms, leading to negative symptoms like sleep disturbance that can trigger depression in vulnerable individuals.
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Mutations in TIM result in an inability to respond to zeitgebers, which is essential for resetting the biological clock. Recent research suggests that, outside the SCN, clock genes may have other important roles as well, including their influence on the effects of drugs of abuse such as cocaine.
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Important environmental cues (zeitgebers) include light, feeding, social behavior, and work and school schedules. Additional research has proposed an evolutionary link between chronotype and nighttime vigilance in ancestral societies. Humans are normally diurnal creatures, that is to say they are active in the daytime. As with most other diurnal animals, human activity-rest patterns are endogenously controlled by biological clocks with a circadian (~24-hour) period.
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The rhythm is linked to the light–dark cycle. Animals, including humans, kept in total darkness for extended periods eventually function with a free-running rhythm. Their sleep cycle is pushed back or forward each "day", depending on whether their "day", their endogenous period, is shorter or longer than 24 hours. The environmental cues that reset the rhythms each day are called zeitgebers (from the German, "time-givers").
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Following up on his temperature studies, he found that a mammalian species can entrain to a temperature cycle, but that temperature is a weak Zeitgeber compared to a light-dark cycle. Aschoff described masking signals as inputs that circumvent the pacemaker but nevertheless lead to modulation of a circadian behavior that is also controlled by the pacemaker.Aschoff J: Masking of circadian rhythms by zeitgebers as opposed to entrainment. In: Trends in Chronobiology, ed.
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The formal study of biological temporal rhythms, such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology. Processes with 24-hour oscillations are more generally called diurnal rhythms; strictly speaking, they should not be called circadian rhythms unless their endogenous nature is confirmed. Although circadian rhythms are endogenous ("built-in", self- sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers (from German, "time giver"), which include light, temperature and redox cycles. In medical science, an abnormal circadian rhythm in humans is known as circadian rhythm disorder.
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The individual's or animal's circadian phase can be known only by the monitoring of some kind of output of the circadian system, the internal "body clock". The researcher can precisely determine, for example, the daily cycles of gene- activity, body temperature, blood pressure, hormone secretion and/or sleep and activity/alertness. Alertness in humans can be determined by many kinds of verbal and non-verbal tests; activity in animals by observation, for example of wheel-running in rodents. When animals or people free-run, experiments can be done to see what sort of signals, known as zeitgebers, are effective in entrainment.
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Biological rhythms, including cycles related to sleep and wakefulness, mood, and cognitive performance, are synchronized with the body's internal circadian clock. The best way to observe the workings of this clock is to experimentally deprive individuals of external cues like light and social interaction and allow the body to experience a "free-running" environment – that is, one in which there are no zeitgebers to influence the body's rhythms. Under these circumstances, the circadian clock alone modulates the body's biological rhythms. Normally however, external cues like light-dark cycles and social interactions also exert an influence on the body's rhythms.
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However, in both unipolar and bipolar depression, the concept of social zeitgebers as potential risk factors has influenced the development of interventions to address this risk. For bipolar disorder, Interpersonal and Social Rhythm Therapy (IPSRT) is meant to regulate and normalize an individual's social rhythms, including meal times, personal relationships, exercise, and social demands. By regulating social rhythms, therapists hope to normalize in turn individuals' biological rhythms. Studies have not found much evidence that IPSRT improves mood, but individuals undergoing IPSRT experience longer periods between bipolar episodes, indicating that normalizing social rhythms may have a preventative effect.
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Seasonal affective disorder may occur as a result of deficiencies in zeitgebers (such as light) during the winter months that stimulate the reward activation system, resulting in a depressed mood. Some studies have pointed to the hormone melatonin, which is regulated by circadian rhythms, as a possible mechanism. Because circadian clocks synchronize human sleep-wake cycles to coincide with periods of the day during which reward potential is highestthat is, during the daytimeand recent studies have determined that daily rhythms in reward activation in humans are modulated by circadian clocks as well, external influences on those rhythms may influence an individual's mood.
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Researchers have attempted to explore the effect life events that disturb social rhythms might have on depressive symptoms in a number of ways. A number of studies have looked at whether the loss of a spouse, a significant negative life event often associated with increased depressive symptoms, might lead to increased depression via disrupted social rhythms. In addition to grief, bereaved spouses may also be dealing with changes in numerous social zeitgebers. For instance, bereaved spouses may suddenly be faced with changes in meal times, responsibilities for additional chores, social expectations, or simply the reality of living day to day without one's usual conversational partner.
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Genes that help control light-induced entrainment include positive regulators BMAL1 and CLOCK and negative regulators PER1 and CRY. A full circadian cycle can be described as a twenty-four hour circadian day, where circadian time zero (CT 0) marks the beginning of a subjective day for an organism and CT 12 marks the start of subjective night. Humans with regular circadian function have been shown to maintain regular sleep schedules, regulate daily rhythms in hormone secretion, and sustain oscillations in core body temperature. Even in the absence of Zeitgebers, humans will continue to maintain a roughly 24-hour rhythm in these biological activities.
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Aschoff provided a strong foundation for the field of chronobiology through his research on circadian rhythms and entrainment in many different organisms such as rats, mice, birds, macaques, monkeys, and humans. His early research focused on understanding the properties of circadian rhythms and how these rhythms can change in response to stimuli. His later work was more applicable to pathologies, such as psychiatric disorders and dangers of shift work schedules, which can result from manipulating specific Zeitgebers. Aschoff's work in the field of chronobiology introduced the idea that shifting one's light-dark cycle can result in harmful effects, such as correlations with mental illness.
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A circadian rhythm is an entrainable, endogenous, biological activity that has a period of roughly twenty-four hours. This internal time-keeping mechanism is centralized in the suprachiasmatic nucleus (SCN) of humans and allows for the internal physiological mechanisms underlying sleep and alertness to become synchronized to external environmental cues, like the light-dark cycle. The SCN also sends signals to peripheral clocks in other organs, like the liver, to control processes such as glucose metabolism. Although these rhythms will persist in constant light or dark conditions, different Zeitgebers (time givers such as the light-dark cycle) give context to the clock and allow it to entrain and regulate expression of physiological processes to adjust to the changing environment.
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Any biological process in the body that repeats itself over a period of approximately 24 hours and maintains this rhythm in the absence of external stimuli is considered a circadian rhythm. It is believed that the brain's suprachiasmatic nucleus (SCN), or internal pacemaker, is responsible for regulating the body's biological rhythms, influenced by a combination of internal and external cues. To maintain clock-environment synchrony, zeitgebers induce changes in the concentrations of the molecular components of the clock to levels consistent with the appropriate stage in the 24-hour cycle, a process termed entrainment. Early research into circadian rhythms suggested that most people preferred a day closer to 25–26 hours when isolated from external stimuli like daylight and timekeeping.
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Therefore, successful entrainment depends on both the melatonin dosage and the appropriate timing of melatonin administration. The accuracy needed for successfully timing the administration of melatonin might require a period of trial and error, as does the dosage. However, entrainment was also observed when giving a dose of melatonin as low as 0.05 mg and without any timing, by just continuing melatonin administration at the same time everyday until the non-24 sufferer's circadian rhythm shifted enough to coincide with melatonin administration (which took from 1 to 2 months). In addition to natural fluctuations within the circadian rhythm, seasonal changes including temperature, hours of daylight, light intensity and diet are likely to affect the efficacy of melatonin and light therapies since these exogenous zeitgebers would compete for hormonal homeostasis.
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Hekkens W, Th JM, Jerkhof GA and Rhietveld WJ, Pergamon Press, Oxford and New York, pp. 149–161, 1988 Parametric entrainment is entrainment that does not result from an instant change in phase, as governed by a Phase Response Curve, as in the case of masking signals. The term Aschoff used for this phenomenon is “arousal” due to non-photic zeitgebers. Data from experimental assays show a relationship between masking effects and phase, leading to a “demasking” effect whereby animals arrhythmic in constant conditions have free-running periods in high frequency light-dark cycles. Aschoff concluded that the oscillator or circadian clock “integrates” over the intensity of light to which it has been exposed, and then responds with a change in the period of activity, as seen in greenfinches, chaffinches, hamsters, and siskins.
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