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Human biological clock: A journey towards circadian rhythm

Human biological clock: A journey towards circadian rhythm

A disruption to the circadian rhythm can affect the heart and impact many bodily functions. Experts recommend melatonin supplements under professional guidance to fix the problem


Like the 24-hour light-dark solar cycle, there is a cycle that sets the physiology and behaviour of an individual. It is called the circadian rhythm. Also called the biological or the internal clock, it lasts for 24.18 hours on average.

Researchers in the sleep health domain have been studying its importance for many years now and it is said to be one of the key components in the human body, which when disrupted can cause harm to many systems.

The review paper, Sleep and Circadian Rhythms in Adolescence, published in the journal, Current Sleep Medicine Reports (2019), reviewed how the biological changes to the regulation of sleep-wake behaviour in adolescence are impacted by environmental factors like the use of digital media and school start times. They summarised that there is a confluence of biological and environmental factors that leads to short and ill-timed sleep among adolescents.

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The biology of the biological clock

Jesse Cook, MS, sleep scientist and a clinical psychology PhD student at the University of Wisconsin-Madison, tells Happiest Health about the vital role of the hypothalamus (present at the centre of the brain) in the circadian rhythm.

“Circadian rhythms regulate the timing of up or down-regulation for most biological processes that play key roles in human wake and sleep. Although there are cellular ‘clocks’ distributed across the body, the suprachiasmatic nucleus of the hypothalamus, or SCN, serves as the principal circadian pacemaker for humans and other mammals. The SCN communicates with subordinate cellular clocks across the body to control behavioural, neuroendocrine and automatic signals,” says Cook.

He points out that over the recent decades, there has been notably improved empirical understanding of the import of the circadian rhythm in its relation to not only sleep health but also other domains of health and functioning including metabolism, physical performance and cognition, among other things.

Among humans, there is variation in one’s biological circadian rhythm, which is labelled as a chronotype. Cook says that the chronotype (an inclination of our body to sleep at a certain time) ranges from extreme morningness to extreme eveningness, with most of us falling in the intermediate range. Circadian preference often aligns with chronotype, and mental and sleep health problems are common consequences of misalignment between chronotype and circadian preference.

There is a wide range of evidence wherein researchers have found that circadian disruptions can affect the whole system. For people with neurological disorders, circadian disruptions are evident. A review published in the Journal of Clinical Investigation says that circadian disruption affects the development, expression and severity of the disease. It also says that there is mounting evidence linking circadian disruption with cerebrovascular disease, epilepsy, migraine, multiple sclerosis, neurodegenerative disorders and neurodevelopmental disorders.

Not just that, the disruption in the circadian rhythm is also common in psychiatric diseases including schizophrenia and mood disorders such as depression, bipolar disorder and seasonal affective disorder. The study lists how it affects the metabolism at the genetic, cellular and system levels, resulting in impaired glucose tolerance and insulin resistance and increased risk of metabolic syndrome and diabetes.

What throws off the rhythm?

“Circadian rhythms can be entrained, with light, physical activity and arousal, and meal timing serving as zeitgebers or clock entrainers. We are most sensitive to light, especially those emitted in the blue wavelength. These factors can be employed to stabilise a circadian rhythm, but also have the potential to negatively affect and disrupt a circadian rhythm,” says Cook.

(Zeitgebers is a German word, meaning ‘time giver’, from Zeit, meaning ‘time’, and Geber, meaning ‘giver’. It refers to any external or intracellular cues that can reset or entrain an organism’s biological rhythms to the 24-hour day-night cycle of Earth.)

Cook says that frequent and chronic exposure to blue light at night or high-intensity exercise at night has the potential to delay the circadian rhythm.

Research has demonstrated that light, especially in the blue light wavelength, has the greatest impact on circadian physiology.

Additionally, sleep-wake schedule regularity is a key factor for circadian rhythm stability. Persons with irregular sleep-wake schedules are often susceptible to circadian rhythm sleep-wake disorders and associated health consequences.

“This can be most saliently seen in persons who have occupations that require shift work, whereby their sleep-wake schedule is often irregular leading to circadian problems. Furthermore, travelling across time zones is another common behaviour that disrupts circadian rhythms. When travelling across time zones, the internal circadian clock no longer matches the external environment, resulting in circadian misalignment. Encouragingly, the body can adapt to a novel time zone, with a general rule of thumb being that 1-hour of circadian adaption occurs with each acclimation day to a new time zone,” he adds.

However, if circadian disruption through time zone changes occurs frequently and without opportunity for adaptation, there is a pronounced increase in the likelihood of degraded functionality and heightened risk for negative health outcomes.

Cook says, “a final point that I would like to draw on in the disruption to circadian rhythms pertains to daylight savings time (For many years, clocks in the countries of the European Union have been put forward by one hour on the last weekend of March, to achieve the daylight saving time). When we shift our clocks, biology does not naturally shift with it. So, the change between daylight savings time and standard time, as we do in many locations around the world, creates a major strain on the circadian system which leads to a significant increase in death on those days. So, that is a direct, tangible effect of circadian disruption.”

Many systems in the body can be affected due to circadian disruptions. It could lead to:

 Cardiovascular disorders

  • Myocardial infarction and stroke
  • Hypertension
  • Arrhythmias

Immunologic disorders

Melatonin and cortisol are regulated by circadian rhythm

While we understand that the circadian rhythm plays a central role in one’s sleep ability and quality, healthy sleep requires good circadian health.

Melatonin and cortisol, two hormones that play key roles in sleep and wake, are tightly regulated by the circadian rhythm. Cook explains, “due to circadian influence, cortisol traditionally begins upregulating in the early morning hours and peaks around late morning or midday before being downregulated. Cortisol exists as a foundational hormone for stimulating wakefulness.”

“In parallel, melatonin is traditionally lowest during periods when cortisol is elevated but is driven by the circadian rhythm to upregulate in the afternoon and evening hours preceding a traditional sleep-wake cycle, with peak melatonin occurring around the midpoint of a traditional sleep window,” he says.

Researchers say sleep problems often emerge when the circadian rhythm is dysregulating these two hormones and/or when a person is attempting to sleep at a time that is not aligned with their chronotype, which often presents as insomnia-related characteristics such as difficulty falling asleep or waking up earlier than desired.

Finding the rhythm

Light, physical activity and arousal, and meal-timing are the three zeitgebers that are leveraged for stabilising or adjusting the circadian rhythm.

“If someone wanted to advance their circadian rhythm, perhaps in a situation where they are taking on a new job that requires waking a few hours earlier, then they would want to expose themselves to a high amplitude bright light immediately upon awakening and across the morning, as well as leverage physical activity and a meal to provide a strong signal of wake to the circadian biology,” says Cook. Additionally, they would want to do the opposite in the hours immediately preceding their targeted bedtime to send a strong signal of sleep to circadian biology. If one was trying to delay their circadian rhythm, then the principles remain the same but in reverse.

“However, delaying the rhythm is much more difficult than advancing the rhythm. Regardless, if you have the intention of doing so, then this is best performed under the guidance of a trained circadian professional,” he adds.

Melatonin supplements only under the guidance of a professional

Explaining the approaches to shifting the circadian rhythm, Cook says, “the use of exogenous (external) melatonin supplements is another way that someone can shift their circadian rhythm. Melatonin is often used to adjust to different disruptions associated with time zone changes. If you were attempting to advance or delay your rhythm in anticipation of an upcoming circadian disruption, then melatonin can be utilised. Yet, again, if you are intending to do this, then it is best performed under the guidance of a trained circadian professional.”

“Ultimately, the name of the game with fixing the circadian rhythm is consistency. Over time, the body will entrain to the sleep-wake lifestyle if it is regular. Yes, it will take time for the biology to change, but constant regularity is at the root of circadian health and change,” he adds.


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