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Circadian rhythms are internal 24-hour cycles that control important body functions and processes, including the well-known sleep-wake pattern. This internal timing mechanism is shaped by environmental factors, most notably light, linking circadian rhythms to the day-night cycle. The circadian rhythm also enables the body to forecast external conditions, such as sunrise and sunset, and to pre-determine optimal periods for sleep, wakefulness, eating, and activity.
The suprachiasmatic nucleus (SCN) in the hypothalamus in the brain is the primary timekeeper for circadian rhythms. The SCN regulates these rhythms at the cellular level and influences various organs, such as the liver, adrenal glands, and the pineal gland.
Our inherent circadian rhythm tends to be longer than a 24-hour cycle, and it varies between individuals. Therefore, it relies on external cues, called zeitgebers or “time-givers,” for constant synchronization with the surrounding environment. Light is the most potent zeitgeber affecting the SCN. Specialized cells in the retina, known as melanopsin-expressing retinal ganglion cells, communicate directly with the SCN. These cells are particularly susceptible to blue light.
Although light remains the predominant zeitgeber, other elements like physical activity and eating patterns can also influence the SCN. For example, morning and early afternoon exercise advances the circadian clock, and evening exercise delays it.
Chronotype is a person’s natural inclination towards a particular daily activity pattern. Depending on the chronotype, you can be a “morning person,” an “evening person,” or somewhere in between, called an “intermediate type.” Your chronotype is largely influenced by the internal circadian rhythms regulated by your biological clock. However, genetic, environmental, and lifestyle factors can also shape it. While chronotype tendencies are often stable traits, they can change throughout a person’s life. For example, children and older adults tend to be more morning-oriented, while teenagers and young adults often shift towards evening types.
The circadian rhythm has allowed organisms to adapt to the predictable changes in their environment that are, in turn, linked to the rotation of the Earth.
Genetics can play a significant role in influencing your circadian rhythm. Specific genes, often called “clock genes,” help regulate the internal mechanisms responsible for your daily sleep cycles, wakefulness, and other physiological processes. Variants in these genes can lead to individual differences in circadian rhythms, including sleep timing, duration, and tolerance to disruptions like jet lag or shift work.
Studies have identified specific genes, such as CLOCK, PER1, PER2, and PER3, closely associated with regulating circadian rhythms. These genes interact in complex ways to maintain the body’s internal clock and can vary in their function from person to person, leading to individual differences. For example, some people are naturally “morning people” and find it easier to wake up early and be productive. In contrast, others are “night owls” whose energy levels peak in the evenings.
Genetic predispositions can also make some individuals more susceptible to circadian rhythm disorders, such as Delayed Sleep Phase Syndrome (DSPS) or Advanced Sleep Phase Syndrome (ASPS). These disorders result in sleep-wake cycles misaligned with societal norms, making it challenging for affected individuals to adhere to conventional schedules.
The molecular clock is an intricate system of genes, proteins, and other cellular components that regulate the circadian rhythm in living organisms. The molecular clock comprises genes like CLOCK, BMAL1 (ARNTL), PER1-3, and CRY1-2 that work together to control time-related processes in the body. The CLOCK and BMAL1 genes help activate other genes, PER1-3 and CRY1-2, which in turn switch off the activity of CLOCK and BMAL1. This creates a loop that helps keep our internal clock running. Some of the most well-studied genes and polymorphisms related to circadian rhythms are described below.
Individuals with the T allele experienced significantly shorter sleep durations compared to those without the allele, even after accounting for socio-economic status, body mass index, alcohol consumption, and smoking habits.
Another widely researched variant of the CLOCK gene with multiple impacts is rs1801260 (also known as CLOCK 3111T/C). Individuals carrying the C allele tend to be more active in the evening, experience delayed sleep initiation, and generally have shorter sleep durations. A study from 2015 analyzed changes in body temperature, activity level, and posture among women who carry this allele. The research discovered that women with the minor allele C showed increased activity in the evening and distinct daily variations in body temperature, leading researchers to conclude that these individuals exhibit a ‘less stable circadian rhythm.’
A genetic variant of the PER2 gene, identified as rs35333999 (p.Val903Ile), was recently linked to chronotype preferences in a comprehensive genome-wide association study (GWAS). This variant is more prevalent among individuals of European descent and is uncommon in African and East Asian groups. The study found that people carrying the minor allele (T) tended to be evening types compared to those with the non-T alleles (C/C).
The PER1 gene produces a protein known as ‘period circadian protein homolog 1.’ This protein, in tandem with CRY (Cryptochrome Circadian Regulator), constitutes the second set of key genes that regulate our body’s internal clock. A 2013 study identified a specific variant, rs7221412, that impacts one’s natural activity timing. Individuals with the A/A genotype are likely to wake up approximately an hour earlier than those with the G/G genotype, with A/G individuals showing an intermediate waking time.
The BMAL1 (ARNTL) gene interacts with CLOCK to enhance the expression of other circadian genes PER and CRY. A specific variant of BMAL1, rs3816358 (with A being the minor allele), has also been investigated for its potential connections to the risk of developing breast cancer, cardiovascular disease, and diabetes.
Several factors can disrupt your circadian rhythm, leading to potential health issues, including sleep disorders, impaired cognitive function, and more. Here’s a list of some everyday disruptors:
The terms “circadian rhythm” and “biological clock” are often used interchangeably, but they refer to distinct yet interconnected concepts in the field of chronobiology.
The “biological clock” refers to an internal timing mechanism that regulates biological processes in an organism. It serves as the master control for generating circadian rhythms and is located in the brain, specifically in the SCN of the hypothalamus in the brain in mammals. This clock is influenced by external cues such as light and temperature, which help synchronize it with the environment. It doesn’t only control sleep-wake cycles but also various other physiological processes like hormone secretion, metabolism, and cell regeneration.
Although the sleep-wake pattern is perhaps the most well-known example of circadian rhythms, these 24-hour cycles are crucial to nearly all bodily systems.
Ongoing research reveals more about circadian rhythms’ role in various health aspects. For instance, studies have linked them to regulating metabolic functions, including blood sugar levels and cholesterol. They are also connected to mental health, affecting the likelihood of depression, bipolar disorder, and even neurodegenerative conditions like dementia.
Emerging evidence suggests that circadian rhythms may significantly impact the immune system and DNA repair mechanisms, critical for cancer prevention. Preliminary studies indicate that the timing of anti-cancer medications may be optimized based on circadian cycles, offering a potential avenue for more effective treatments.
When your circadian rhythm is out of sync, the body’s internal systems fail to operate at their peak efficiency. One of the most immediate consequences is a disruption in the sleep-wake cycle. Absent the proper cues from the internal clock can reduce the total amount of sleep, low-quality, fragmented sleep, and increase the risk of insomnia and excessive daytime fatigue.
Research indicates circadian rhythm imbalances may also be implicated in specific sleep disorders, such as obstructive sleep apnea (OSA). This disorder is characterized by repeated interruptions in breathing during sleep, leading to decreased oxygen levels and multiple awakenings throughout the night.
Maintaining a healthy circadian rhythm is essential for optimal physical and mental well-being. Here are some tips to keep your circadian rhythm in control:
Sleep is essential for everyone’s health and well-being. LifeDNA’s Sleep Report gives an insight into several traits that can help you optimize your sleep at night.
LifeDNA’s Sleep report covers an analysis of Circadian Rhythm (Chronotype). Get yours here.
*Understanding your genetics can offer valuable insights into your well-being, but it is not deterministic. Your traits can be influenced by the complex interplay involving nature, lifestyle, family history, and others.
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