A zeitgeber () is any external or environmental cue that entrains or synchronizes an organism's
biological rhythms, usually naturally occurring and serving to entrain to the Earth's 24-hour light/dark and 12-month cycles.
History
The term ' (; ) was first used by
Jürgen Aschoff
Jürgen Walther Ludwig Aschoff (January 25, 1913 – October 12, 1998) was a German physician, biologist and behavioral physiologist. Together with Erwin Bünning and Colin Pittendrigh, he is considered to be a co-founder of the field of chrono ...
, one of the founders of the field of
chronobiology
Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobi ...
. His work demonstrated the existence of
endogenous
Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell.
In contrast, exogenous substances and processes are those that originate from outside of an organism.
For example, es ...
(internal) biological clocks, which synchronize biological rhythms. In addition, he found that certain
exogenous
In a variety of contexts, exogeny or exogeneity () is the fact of an action or object originating externally. It contrasts with endogeneity or endogeny, the fact of being influenced within a system.
Economics
In an economic model, an exogeno ...
(external) cues, which he called zeitgeber, influence the timing of these internal clocks.
Photic and nonphotic zeitgebers
* Light (light is a more important zeitgeber than social interactions).
* 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
A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogeno ...
.
It is believed that the brain's
suprachiasmatic nucleus
The suprachiasmatic nucleus or nuclei (SCN) is a tiny region of the brain in the hypothalamus, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regula ...
(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
Entrainment may refer to:
* Air entrainment, the intentional creation of tiny air bubbles in concrete
* Brainwave entrainment, the practice of entraining one's brainwaves to a desired frequency
* Entrainment (biomusicology), the synchronization o ...
.
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. However, this research was faulty because it failed to shield the participants from artificial light. Although subjects were shielded from time cues (like clocks) and daylight, the researchers were not aware of the phase-delaying effects of indoor electric lights. The subjects were allowed to turn on light when they were awake and to turn it off when they wanted to sleep. Electric light in the evening delayed their circadian phase; these results became well known.
[ More recent research has shown that adults have a built-in day, which averages about 24 hours; indoor lighting does affect circadian rhythms; and most people attain their best-quality sleep during their ]chronotype
A chronotype is the behavioral manifestation of underlying circadian rhythm's myriad of physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period. ''Eveningness'' (delayed ...
-determined sleep periods. A study by Czeisler et al. at Harvard found the range for normal, healthy adults of all ages to be quite narrow: 24 hours and 11 minutes ± 16 minutes. The "clock" resets itself daily to the 24-hour cycle of the Earth's rotation.
Biological rhythms, including cycles related to sleep and wakefulness, mood, and cognitive performance, are synchronized with the body's internal circadian clock
A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase (waves), phase and is synchronized with solar time.
Such a clock's ''in vivo'' period is necessarily almost exactly 24 hours (the earth's curre ...
. 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. These zeitgebers do so by alerting individuals to changes in the likelihood of possible rewards
Reward may refer to:
Places
* Reward (Shelltown, Maryland), a historic home in Shelltown Maryland
* Reward, California (disambiguation)
* Reward-Tilden's Farm, a historic home in Chestertown Maryland
Arts, entertainment, and media
* "Reward" ...
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. For example, Jürgen Aschoff
Jürgen Walther Ludwig Aschoff (January 25, 1913 – October 12, 1998) was a German physician, biologist and behavioral physiologist. Together with Erwin Bünning and Colin Pittendrigh, he is considered to be a co-founder of the field of chrono ...
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
Jet lag is a physiological condition that results from alterations to the body's circadian rhythms caused by rapid long-distance trans-meridian (east–west or west–east) travel. For example, someone flying from New York to London, i.e. from ...
, 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
Working memory is a cognitive system with a limited capacity that can hold information temporarily. It is important for reasoning and the guidance of decision-making and behavior. Working memory is often used synonymously with short-term 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. In addition, individual differences among participants can have an effect on daily rhythms in performance. Studies have found that children perform mental math exercises most successfully in the morning,[Rutenfranz J, Helbruegge T. 1957. Über Tageschwankungen der Rechengeschwindigkeit bei 11-jährigen Kindern. Z Kinderheilk. 80:65–82] 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. Based on these findings, researchers conclude that factors that disturb circadian rhythms can also affect cognitive performance.
Mood disorders
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
Psychopathology is the study of abnormal cognition, behaviour, and experiences which differs according to social norms and rests upon a number of constructs that are deemed to be the social norm at any particular era.
Biological psychopatholo ...
. There is strong evidence that individuals with depression experience irregular biological rhythms, including disrupted sleep-wake cycles, temperature, and cortisol
Cortisol is a steroid hormone, in the glucocorticoid class of hormones. When used as a medication, it is known as hydrocortisone.
It is produced in many animals, mainly by the ''zona fasciculata'' of the adrenal cortex in the adrenal gland ...
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. Recent work has also demonstrated that interventions like light therapy
Light therapy, also called phototherapy or bright light therapy is intentional daily exposure to direct sunlight or similar-intensity artificial light in order to treat medical disorders, especially seasonal affective disorder (SAD) and circadi ...
, sleep deprivation, and some pharmacological antidepressants may be effective in treating depression by reordering these rhythms to their natural state. Such interventions influence an individual's mood, body temperature, cortisol levels, and melatonin
Melatonin is a natural product found in plants and animals. It is primarily known in animals as a hormone released by the pineal gland in the brain at night, and has long been associated with control of the sleep–wake cycle.
In vertebrates ...
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. This possibility may help to explain the relationship between stressful life events and the development of mood disorders. For example, newly married or cohabiting couples often need to adjust to each other's sleeping rhythms when beginning to share the same bed for the first time. This adjustment can be difficult and may lead to disruptions in sleep quality and quantity, and possibly increase risk for depression as a result. 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. Taken together, findings from studies on bereaved spouses indicate that when bereavement is associated with changes in social rhythms, depressive symptoms are likely to increase; however, if bereaved individuals are able to maintain social rhythms after the death of their spouse, increased depression is less likely. These findings suggest that social rhythm stability may not be entirely dependent on life events, but rather has some traitlike elements as well, since some individuals may be more likely to maintain social rhythms than others following the occurrence of a significant life event.
Recent studies have also found a connection between the disruption of social rhythms and the triggering of manic episodes in bipolar disorder
Bipolar disorder, previously known as manic depression, is a mental disorder characterized by periods of depression and periods of abnormally elevated mood that last from days to weeks each. If the elevated mood is severe or associated with ...
. 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
Interpersonal and social rhythm therapy (IPSRT) is an intervention for people with bipolar disorder (BD). Its primary focus is stabilizing the circadian rhythm disruptions that are common among people with bipolar disorder (BD). IPSRT draws upon p ...
(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.
Seasonal affective disorder
Seasonal affective disorder
Seasonal affective disorder (SAD) is a mood disorder subset, in which people who have normal mental health throughout most of the year exhibit depressive symptoms at the same time each year.
Common symptoms include sleeping too much, having li ...
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.
See also
* Infradian rhythm
In chronobiology, an infradian rhythm is a rhythm with a period longer than the period of a circadian rhythm, i.e., with a frequency of less than one cycle in 24 hours. Some examples of infradian rhythms in mammals include menstruation, breeding, m ...
* Phase response curve
A phase response curve (PRC) illustrates the transient change (phase response) in the cycle period of an oscillation induced by a perturbation as a function of the phase at which it is received. PRCs are used in various fields; examples of biolog ...
* Ultradian
In chronobiology, an ultradian rhythm is a recurrent period or cycle repeated throughout a 24-hour day. In contrast, circadian rhythms complete one cycle daily, while infradian rhythms such as the human menstrual cycle have periods longer than ...
References
{{reflist
Circadian rhythm
Sleep physiology