EVERYTHING ABOUT CIRCADIAN RHYTHMS
Did you know that your body has an in-built clock that not only tells you when to wake and sleep but influences hormone release, metabolic function and cognition?
Despite our circadian rhythms being one of the most important mechanisms in our body, they largely go unnoticed even though understanding them could be the key to achieving better health, productivity, and emotional well-being.
In a time of seemingly inescapable technology, night shifts and constantly changing lifestyles, have we left it too late to restore the delicate balance of our circadian rhythms and save our sleep quality, mood, and health?
Let’s dive into everything and more about circadian rhythms to find out.
What Do Circadian Rhythms Do?
Circadian rhythms are essential biological processes that regulate sleep-wake cycles, hormone releases, body temperature and other physiological functions. [2] [3]
These physiological processes include cell proliferation (cell division and growth), DNA repair, metabolism, redox state (oxidisation-reduction state), immune response, and inflammation. [28]
The timing of sleep is determined by an interaction between the circadian system and homeostatic sleep drive. The circadian system provides a wake-promoting signal during the biological day, while homeostatic pressure accumulates during wakefulness, creating the drive to sleep. [29]
The function of sleep is still not entirely understood but it is necessary for neural maturation and synaptic plasticity, memory consolidation, waste clearance in the brain, and metabolic energy conservation. [1]
How Do Circadian Rhythms Work?
The suprachiasmatic nucleus in the hypothalamus acts as the master clock for the body. It receives light input via the retina through the retinohypothalamic tract. The suprachiasmatic nucleus uses these light signals to coordinate the body’s internal clock with the external light-dark cycle. [2]
Circadian rhythms are maintained by a molecular clockwork system consisting of several clock genes. These genes interact in transcription-translation feedback loops, maintaining rhythmic cycles of gene expression and physiological functions. [2] [28]
What Influences Circadian Rhythms?
Circadian rhythms are influenced by external cues known as Zeitgebers (translated to ‘time givers’ in German) such as light, meal timing, diet, and exercise. These signals regulate and synchronise the body’s internal clock with the environment. [11]
The Earth’s electromagnetic fields and Schumann resonances (electromagnetic waves that interact with human brainwaves) may also act as Zeitgebers, however further research is needed to understand its complex interaction with circadian rhythms. [14]
Circadian rhythm disruptions, often due to factors such as artificial light, irregular meal timing, night shift work, and irregular sleep patterns are linked to adverse health outcomes. [6]
Artificial Light
Light exposure is the primary cue for regulating circadian rhythms. [21]
When light enters our eyes, photoreceptors in our retina detect changes in light intensity, and signals to the suprachiasmatic nucleus how to regulate the secretion of melatonin (a sleep-inducing hormone). [21] [29] [31]
Put simply, light indicates to our bodies when it is time to wake and sleep.
Studies have shown that bright light exposure during the day and reduced light exposure in the evening help align our circadian rhythms with our external environment. [21] [29]
Conversely, studies have also proven that artificial lighting such as blue light omitted from technology is delaying the release of melatonin and disrupting natural day-night cycles, resulting in circadian misalignment. [18] [21] [29]
The result of this is not just negatively affected sleep quality but an increased risk of sleep disorders, mood episodes, altered stress responses, cardiovascular disease and various metabolic disturbances. [2] [18] [21]
Chrono Nutrition
The concept of chrono-nutrition refers to the impact of meal timing on metabolic and health outcomes, having been identified as a key influence over peripheral clocks (biological clocks throughout the body). [15] [22]
Eating meals during periods of elevated melatonin (at night) can impair glucose metabolism, as melatonin reduces insulin sensitivity, which may lead to higher blood sugar levels after evening meals. [24]
Several randomised controlled trials have demonstrated that eating earlier in the day initiates the transcription of genes involved in glucose uptake and energy metabolism, reducing blood glucose, insulin levels, and body weight. [24]
One study compared ad libitum eating (continuous access to food) with time-restricted eating (food available only during a specific window) and found that time-restricted eating strengthens circadian rhythms in the gut.
This enhances the cycling of genes related to metabolic functions, such as energy production and amino acid metabolism. [23] [24]
Numerous studies have shown that late-night eating can disrupt the body’s internal clock, impairing metabolic processes like glucose regulation and lipid metabolism, leading to a higher risk of metabolic disorders, weight gain, and reduced insulin sensitivity. [15] [18] [22] [24]
Diet
As well as meal timing, diet can affect circadian rhythms.
High-fat diets are identified as potential chrono-disruptors, negatively impacting metabolic regulation. In contrast, diets rich in antioxidants and tryptophan support sleep quality and recovery. [26]
Whereas carbohydrate intake affects insulin release and can enhance the transport of tryptophan to the brain, boosting melatonin production and improving sleep. [26]
Conversely, dietary interventions can modulate the gut microbiome to disrupt or restore circadian rhythm, pointing to the dynamic interplay between diet, circadian rhythms, and microbial populations. [15]
Exercise
Exercise is highlighted as having significant influence on circadian rhythms.
Both aerobic and resistance exercises upregulate the activity of core clock genes, promoting a properly functioning biological clock. [17]
Core clock genes such as BMAL1 and PER2 influence other genes and physiological processes throughout the body which are crucial for maintaining metabolic, cardiovascular, and immune health. [17]
One review emphasises that regular exercise can reset or synchronise our internal biological clock, making it a potential intervention for circadian rhythm disorders. [17]
Exercise timing also influences melatonin production and core body temperature. Morning exercise has been shown to advance melatonin onset in certain chronotypes, while evening exercise may delay melatonin onset, potentially disrupting the body’s internal clock. [17]
Why Are Circadian Rhythms Important?
Disruptions to circadian rhythms are strongly associated with mood disorders including Seasonal Affective Disorder, Major Depressive Disorder, and Bipolar Disorder. [2] [7]
Monoamine neurotransmitters like dopamine and serotonin are tightly regulated by circadian rhythms. Abnormalities in dopamine signalling are linked to manic episodes in bipolar disorders. [2]
Up to one-third of people with bipolar disorder have a diagnosable circadian rhythm disorder, with delayed sleep phase disorder being the most common. [2]
One study demonstrated that as aligned with circadian rhythms, worser mood levels are felt more strongly during morning hours (associated with the lowest core body temperature) and measures of wellbeing showed improvement throughout the day. [8]
Neurodegenerative Diseases
Neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, and Huntington’s disease share common characteristics which are exacerbated by circadian rhythm disruptions. [3]
A growing body of evidence suggests that sleep disturbances and circadian rhythm disruptions are both a symptom and a potential contributing factor to neurodegenerative diseases. [20] [30]
Cardiovascular System
Disruptions to circadian rhythms desynchronises that master and peripheral clocks, affecting cardiovascular functions and increasing the incidence of acute myocardial infarction (heart attack), sudden cardiac death, and stroke. [21]
Blood pressure follows a natural circadian rhythm, characterised by higher blood pressure during the day (wakefulness) and lower blood pressure at night (sleep). [19] [21]
Disruptions to these circadian rhythms can exacerbate hypertension due to factors such as renal salt handling and changes in catecholamine production (both regulate blood pressure). [12] [19]
Musculoskeletal System
The musculoskeletal system’s clocks depend on consistent eating and physical activity. Circadian rhythm disruptions can lead to musculoskeletal disturbances, increasing the risk of bone fractures, osteoporosis, and muscle atrophy. [16]
Immune System
Disruptions in circadian regulation can impair immune cell metabolism, reducing their effectiveness in responding to pathogens (disease-causing microorganisms) and inflammation. [10]
The immune system operates in a rhythmic pattern, with Core clock genes regulating mitochondrial activity, energy production, and the generation of reactive oxygen species, which are crucial for immune cell functions. [10] [25] [26]
Where Do Chronotypes Come In?
Chronotypes refer to individual preferences for activity at different times of the day.
Several studies confirm that people with an evening chronotype are especially prone to mood disorders and depressive episodes, due to their natural tendency to stay up late, resulting in less exposure to natural daylight. [2]
Evening chronotypes are more prone to experiencing irregular social rhythms, sleep irregularities, and delayed sleep phase disorder. [9]
is There Any Difference Between The Sexes?
Women and men vary in a number of factors such as hormonal influences, circadian rhythm speeds, and chronotype variations which mean that compared to men, women exhibit an earlier circadian phase and have greater sleep efficiency. [4] [31]
This earlier circadian phase is thought to contribute to sleep disorders, such as insomnia, which are more prevalent among women. It also means that early time-restricted eating and exercising in the morning benefits women more. [4] [26]
Another difference is that females tend to have higher body temperatures across the circadian day and generally have higher resting and active heart rates compared to males, whereas men exhibit higher systolic and diastolic blood pressure. [31]
How Is Athletic Performance Affected?
One study highlighted that sports performance peaks in the afternoon, coinciding with peak core body temperature, which enhances muscle function and metabolic efficiency. Physical activities performed in the afternoon lead to better mental performance, reaction times, and muscle strength. [26]
Anaerobic performance (such as short bursts of high-intensity exercises) is most effective in the afternoon, but aerobic exercises show mixed findings, indicating that regular training might improve morning performance. [26]
Naturally, adequate sleep is essential for tissue repair, cognitive function, and recovery, especially during deep sleep stages like slow-wave sleep. [26]
What Can I Do?
Get enough sleep – aim for 7 to 9 hours per night and adhere to a consistent sleep schedule to avoid social jet lag.
Morning sunlight exposure – getting early daylight exposure is crucial for entraining circadian rhythms.
Reduce evening light exposure – limit your screen time in the evening to promote melatonin production and circadian alignment.
Increase light exposure during the day – especially for those working indoors, make an effort to face a window or go for a walk outside on your break.
Eat earlier – eating breakfast and avoiding late dinners aligns food intake with the body’s natural circadian rhythm, leading to improvements in glucose control and fat oxidation. [24]
References
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