Zeitgeber

The term '''' () was first used by Jürgen Aschoff, one of the founders of the field of chronobiology. His work demonstrated the existence of endogenous (internal) biological clocks, which synchronize biological rhythms. In addition, he found that certain exogenous (external) cues, which he called zeitgeber, influence the timing of these internal clocks. ==Photic and nonphotic zeitgebers==

• Light, the most potent regulator of circadian rhythms • Atmospheric conditions • Medication • Temperature • Social interactions • Exercise • Eating/drinking patterns ==Circadian rhythms==

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. 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. In this experiment, humans were in an underground, sound-proof bunker where the lights were continuously on. The subjects prepared their own meals and took their own samples, so there were no social interaction cues either. Humans therefore adjust from 24.9 hr internal rhythm to the 24-hour day. There are many different zeitgebers, and their relative influence on an individual at any given time depends on a number of factors, time of day the zeitgeber is presented and the intensity of that zeitgeber. 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. ==Psychological effects of changes==

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. Cognitive performance Researchers have shown that the 24-hour circadian clock also influences cognitive performance in a wide variety of paradigms, including serial search, verbal reasoning, working memory tasks, suppressing wrong answers, and manual dexterity. Performance on these tasks varies over the course of a day, with each type of task having a unique daily rhythm. For example, the best time to perform a working memory task tends to be midday, while immediate memory is best in the morning, and simple processing is ideally performed in the evening. but young adults' performance peaks in the evening. This variation in the performance of various tasks is attributable to a number of factors, including relative working memory load, change in strategy, hemispheric dominance, ability to suppress wrong answers, age, level of practice, and morningness-eveningness, many of which fluctuate according to time of day. 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. Such interventions influence an individual's mood, body temperature, cortisol levels, and melatonin production, all of which appear to be irregular in depressed individuals. Social zeitgebers and mood disorders Some researchers have suggested that the disturbances in biological rhythms present in depressed individuals may actually be the result of previous disruptions in social interactions, which serve as cues for those rhythms. Differentiating between zeitgeber disturbances that lead to depression and those that lead to manic episodes has proven difficult. 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. 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. ==See also==