Human Biological Clock

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Human Biological Clock Sept 12, 2021 20:40:53 GMT

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Post by Admin on Sept 12, 2021 20:40:53 GMT

What are circadian rhythms?
Circadian rhythms are physical, mental, and behavioral changes that follow a 24-hour cycle. These natural processes respond primarily to light and dark and affect most living things, including animals, plants, and microbes. Chronobiology is the study of circadian rhythms. One example of a light-related circadian rhythm is sleeping at night and being awake during the day. The Average Teen Circadian Cycle image shows the circadian rhythm cycle of a typical teen.

What are biological clocks?
Biological clocks are organisms’ natural timing devices, regulating the cycle of circadian rhythms. They’re composed of specific molecules (proteins) that interact with cells throughout the body. Nearly every tissue and organ contains biological clocks. Researchers have identified similar genes in people, fruit flies, mice, plants, fungi, and several other organisms that make the clocks’ molecular components.

What is the master clock?
A master clock in the brain coordinates all the biological clocks in a living thing, keeping the clocks in sync. In vertebrate animals, including humans, the master clock is a group of about 20,000 nerve cells (neurons) that form a structure called the suprachiasmatic nucleus, or SCN. The SCN is in a part of the brain called the hypothalamus and receives direct input from the eyes.

Does the body make and keep its own circadian rhythms?
Yes, natural factors in your body produce circadian rhythms. For humans, some of the most important genes in this process are the Period and Cryptochrome genes. These genes code for proteins that build up in the cell’s nucleus at night and lessen during the day. Studies in fruit flies suggest that these proteins help activate feelings of wakefulness, alertness, and sleepiness. However, signals from the environment also affect circadian rhythms. For instance, exposure to light at a different time of day can reset when the body turns on Period and Cryptochrome genes.

How do circadian rhythms affect health?
Circadian rhythms can influence important functions in our bodies, such as:

Hormone release
Eating habits and digestion
Body temperature

However, most people notice the effect of circadian rhythms on their sleep patterns. The SCN controls the production of melatonin, a hormone that makes you sleepy. It receives information about incoming light from the optic nerves, which relay information from the eyes to the brain. When there is less light—for example, at night—the SCN tells the brain to make more melatonin so you get drowsy.

What factors can change circadian rhythms?
Changes in our body and environmental factors can cause our circadian rhythms and the natural light-dark cycle to be out of sync. For example:

Mutations or changes in certain genes can affect our biological clocks.
Jet lag or shift work causes changes in the light-dark cycle.
Light from electronic devices at night can confuse our biological clocks.

These changes can cause sleep disorders, and may lead to other chronic health conditions, such as obesity, diabetes, depression, bipolar disorder, and seasonal affective disorder.

How are circadian rhythms related to jet lag?
When you pass through different time zones, your biological clock will be different from the local time. For example, if you fly east from California to New York, you “lose” 3 hours. When you wake up at 7:00 a.m. on the East Coast, your biological clock is still running on West Coast time, so you feel the way you might at 4:00 a.m. Your biological clock will reset, but it will do so at a different rate. It often takes a few days for your biological clock to align with a new time zone. Adjusting after “gaining” time may be slightly easier than after “losing” time because the brain adjusts differently in the two situations.

How do researchers study circadian rhythms?
Scientists learn about circadian rhythms by studying humans and by using organisms with similar biological clock genes, such as fruit flies and mice. Researchers doing these experiments control the subject’s environment by altering light and dark periods. Then they look for changes in gene activity or other molecular signals. Scientists also study organisms with irregular circadian rhythms to identify which genetic components of biological clocks may be broken.

Understanding what makes biological clocks tick may lead to treatments for jet lag, sleep disorders, obesity, mental health disorders, and other health problems. It can also improve ways for people to adjust to nighttime shift work. Learning more about the genes responsible for circadian rhythms will also help us understand more about the human body.

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Human Biological Clock Sept 13, 2021 3:03:58 GMT

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Post by Admin on Sept 13, 2021 3:03:58 GMT

The human circadian clock entrains to sun time
Till Roenneberg
C. Jairaj Kumar
Martha Merrow

DOI:https://doi.org/10.1016/j.cub.2006.12.011

The human biological clock, which regulates processes from gene expression to behaviour, like that of most organisms, synchronises to the Earth's 24 hour rotation using signals from the environment (zeitgebers). This synchronisation is an active process called entrainment [1]. Daily rhythms also persist in temporal isolation [2], deviating modestly from 24 hours (hence circadian). Light is the strongest zeitgeber for all circadian systems, yet social cues also entrain human clocks [3], possibly via behaviour, for example changing light exposure by closing eyes during sleep [4, 5]. Here, we ask what zeitgebers entrain the human clock in real life by exploiting the common discrepancy between social time and sun time. Our results show that the human clock entrains to sun time.

Within a given time zone, people live according to a common social time — which tells them, for example, when to go to work or watch the evening news. Dawn and dusk, however, progress from East to West, creating a continuum in sun time. This creates discrepancies between, for example, the actual mid-dark phase and midnight according to local clock time. By definitions of the time zones, mid-dark and midnight coincide in London or in any other place that is a multiple of 15° longitude East or West of Greenwich (mid-dark and midnight precisely coincide only twice a year; mid-dark undulates around midnight, deviating by approximately ± 15 minutes, independently of location). Accordingly, midnight occurs almost one hour before mid-dark in Paris and more than 90 minutes earlier in Santiago de Compostela. In orienting ourselves predominantly according to the social clock, we appear to be oblivious to these discrepancies. This raises the question as to whether the human clock is an exception to the rules determined for other circadian systems, which are predominantly entrained to light. If humans were entrained by social time, average sleep-wake behaviour should not change from East to West, while a gradual change, or at least some systematic deviation from time zone constancy, should be apparent if the human clock is (also) influenced by dawn and dusk.

We determine sleep-wake-behaviour by assessing chronotype with a dedicated questionnaire (MCTQ). It asks people for their habitual activity and rest times on work and free days [6, 7, 8], and calculates chronotype by using the mid-sleep times on free days, corrected for sleep debt accumulated over the work week (see Supplemental data available on-line with this issue). For genetic and epidemiological studies, such as this one, further normalisations for age and sex are applied. Chronotype reflects how individual circadian clocks are entrained within the 24 hour day — earlier or later. Partly because of genetic variation, chronotypes form a normal distribution in a population ranging from extreme early to extreme late types [7]. The MCTQ database currently comprises more than 40,000 individuals; for this study, we used the 21,600 responses which contained a German postal code and the correct name of the corresponding location to unambiguously allow geographical mapping (Figure 1A). The sample's sex ratio is identical to that in the German population (51% women), and the number of entries for each location highly correlates with its population size (r = 0.96). Entries were grouped according to population size (300,000; 300,001–500,000; >500,000) based on the hypothesis that — in general — exposure to natural light decreases statistically the bigger the city thereby weakening the strength of this zeitgeber. Within the first group, chronotype (averaged in longitudinal bins) is tightly coupled to sun time (Figure 1B), while those in larger and big cities (averaged by city) show a progressively weaker coupling (Figure 1C).

Figure 1
Chronotype and place of residence.

(A) The geographical distribution of the German places of residence of individuals contributing to this study (N = 21,600). A map of Germany is shown by the small grey insert. (B) The dependence on longitude of normalised chronotype of people living in areas with up to 300,000 inhabitants. representing 82% of the German population (for normalisation, see Supplemental data; chronotypes are averaged within longitudinal bins spanning the West–East extension of Germany; regression line: r = 0.962; p < 0.0001; slope −34.2 min per 9°). (C) The dependencies of average chronotype on longitude for 9 towns with populations between 300,001 and 500,000 (grey symbols; r = 0.934; p < 0.001; slope −24.2 min per 9°) and 11 cities with more that 500,000 inhabitants (black symbols; r = 0.703; p = 0.010; slope −13.6 min per 9°). All correlations are also highly significant for the respective raw, un-binned datasets. For reference, the slopes of the shaded areas in (B,C) reflect approximate differences in dawn times at the different longitudes on June 21st (the difference between longitude 6° and 15° is 36 min). Standard error of the mean is present for all data points but often is smaller than the size of the dots.

Last Edit: Sept 13, 2021 3:11:00 GMT by Admin

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Human Biological Clock Sept 13, 2021 19:18:44 GMT

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Post by Admin on Sept 13, 2021 19:18:44 GMT

Our results strongly suggest that the human circadian clock is predominantly entrained by sun time rather than by social time. We have deliberately selected only German residents for this study to prevent possible confounding influences of cultural differences. It could be argued that post-war Germany was separated into two states of different cultural influence until 1989. However, the same slope (determined as for Figure 1B) is also found for locations in Southern Germany alone, which were not part of the former German Democratic Republic (correlation between the results for all of Germany and Southern Germany alone: r = 0.99). These results exclude potential socio-cultural influences on the gradual changes of sleep-wake behaviour from East to West. They also indicate that the observed phenomenon, entrainment to sun time, is independent of latitude.

The question remains why inhabitants of large cities show, on average, a later chronotype and a less stringent correlation with the East–West progression of the sun. It has to be noted that 82% of Germans live outside of the 20 cities with more than 300,000 inhabitants. On a European scale, this includes cities such as Bordeaux, Innsbruck, Leicester, Pamplona, Porto or Venice. That inhabitants of even larger cities are less coupled to sun time could be due to less exposure to outdoor light, resulting in a weaker zeitgeber strength of the natural light–dark cycle. Light and darkness even play a role when humans are entirely entrained by social cues: when we sleep, we close our eyes and in most cases, avoid light.

That social cues alone cannot entrain the human circadian clock — without concurrent (behavioural) light changes — is evident in blind people without any residual vision: their clocks run free with their own non-24 hour periodicity in spite of regular work schedules [9]. Normally, the darkness caused by behaviour coincides more or less with the environmental night. In shift workers who have to sleep during the day but who cannot completely escape the natural light–dark cycle (for example, on their way to and/or from work and in their free time), two light–dark-cycles compete in the entraining process. As a result, these workers have difficulties entraining to their work schedules while, for example, shift workers on oil rigs who only alternate between their night-work and their enclosed cabins have no difficulties [10]. The gradual uncoupling of the circadian clock from sun time of people living in large cities may reflect the gaining strength of the behavioural light–dark cycle as the strength of the environmental light–dark cycle decreases. Whenever the latter becomes more dominant, the overall zeitgeber strength will be weaker as predicted for people in metropolitan hubs. Weaker zeitgebers also predictably lead to later chronotypes [1, 7], and our data show that chronotype is progressively delayed with increasing population size.

Our findings emphasise the importance of individual, circadian time rather than social, external time in scientific studies, in school and work schedules or in medical considerations. They also demand careful re-examination of how changing to and from daylight saving time affects individuals [11, 12].

Supplemental data

www.cell.com/cms/10.1016/j.cub.2006.12.011/attachment/a2777217-569e-40e5-bb0a-04bce8b49610/mmc1.pdf

Last Edit: Sept 13, 2021 19:19:08 GMT by Admin