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Why is a regular circadian rhythm so important for health?
M.Sc Exercise Science (Exercise Physiology), University of Jyväskylä
M.Sc Exercise Science (Exercise Physiology), University of Jyväskylä
Written by Julius Granlund & Tapio Tulenheimo.
In this article we explain the physiological and psychological consequences of circadian rhythm disruption and the mechanisms behind them. Future articles will focus on practical actions to easily optimize the circadian rhythm, and therefore your body's physiological functioning.
Why is circadian rhythm so important?
Most of our body's functions are paced according to a biological rhythm of about 24 hours (1). This biological rhythm regulates important physiological processes such as metabolism, mood, cognitive function, body temperature, immune function and hormonal activity (2, 3, 4, 5, 6, 7). Evolutionarily, this has been inevitable, as humans have been subjected to this rhythm throughout their existence, as the Earth has rotated on its axis every 24 hours, causing constant daily changes in light, temperature and food availability. Humans have adapted to being active during the day and resting at night - this has been beneficial for survival (8). This helps to explain why deviating from biological rhythms can be so detrimental - for at least hundreds of thousands of years, the human body has adapted to act in a certain way at sunrise and sunset (9). On the other hand, understanding and optimizing biological rhythms is extremely beneficial to health.
In today's society, it is easy for your circadian rhythm to get disrupted or working suboptimally. This means that the body's biological circadian rhythm is irregular, does not correspond to the environmental circadian rhythm, and many of the body's functions are not functioning optimally. This can easily happen, for example, by seeing too much bright light in the evening, getting too little light in the morning, and eating or exercising at inappropriate times (10).
These seemingly small factors, or for example shift work, can cause biological rhythms to become out of sync, leading to jet lag symptoms (11). Common symptoms include fatigue, mood swings, sleep disturbance, nausea, digestive problems, cognitive problems, memory impairment and general confusion (11, 12, 13). However, the effects are much greater than simply being tired at the wrong time of day. Underlying this is a wider physiological and psychological disruption that can have major health consequences, including an increased risk of several diseases (TABLE 1).
TABLE 1. Disruption of circadian rhythms is associated with increased risk for at least the following physiological and psychological diseases and negative outcomes. (It should be noted that the data comes mainly from shift workers and is correlative, but there's also evidence at the mechanism level for many diseases. In addition, circadian rhythms and sleep are closely linked. The table lists studies that mainly focus on the effect of circadian rhythm, but sleep is also intrinsically linked to the findings and is often difficult to control for, although some studies are able to distinguish between the two. If the list included diseases and disorders directly related to sleep, the list would be much longer.)
PHYSIOLOGICAL
- Metabolic syndrome (2022, 2021,2009, 2011, 2009, 2010)
- Risk of premature death (2009, 2010)
- Obesity (2018, 2013,2018)
- Type II diabetes (2022, 2020)
- Conflicting evidence on cancer risk (2021)
- Atherosclerosis (2020, 1999)
- Coronary heart disease (2003, 1995, 2019)- Myocardial infarction (2019, 1995)
- Cardiac arrhythmias (2019)
- Hypertension (2021, 2012)
- Menstrual disorders (2019, 2007, 2010)
- Reproductive disorders in women (2010, 2003, 2013) and some evidence also in men (2022, 2019,2022)
- Impaired insulin sensitivity (2020, 2019)
- Impaired fat metabolism (2016)
- Multiple abdominal disorders, e.g. constipation, abdominal pain (2020), IBS (2018), stomach ulcers (2010)
- Impaired immune system (2021, 2020)
- Impaired secretion of various hormones (2012)
PSYCHOLOGICAL
- Bipolar disorder (2018, 2017)
- Different types of depression (2020, 2018, 2017, 2016, 2016)
- Mood swings (2018)
- Perceived loneliness (2018)
- Reduced perceived happiness and quality of life (2018)
- Reduced reaction time (2018)
- Anxiety, although this could be more related to disturbed sleep (2020)
- Temporal lobe atrophy (2001)
- Impaired concentration (2019)
- Accidents (2016)
- Increased number of errors at work (2016)
- Impaired work performance (2016)
- Sleep disturbance (2019)
- Drowsiness (2014)
How is the biological circadian rhythm regulated?
The body's biological circadian rhythm is regulated by an internal clock that gets paced by cues from the environment (so-called "Zeitgebers"). The main environmental cue is light exposure, but the timing of diet and exercise also play an important role (15). When the circadian rhythm remains constant daily and the environmental cues are correctly timed, optimal functioning of the body is ensured. So how does this internal clock work? The circadian clock is essentially a regulatory mechanism at two different levels: the "central clock" in the brain (the suprachiasmatic nucleus, SCN) and the "clock genes" found throughout the body. Together, these regulate the internal timing of the whole body.
The figure below illustrates some of the important organs and functions regulated by the internal clock. In fact, clock genes affect almost every cell in the body (16).
Figure 3. Clock genes act throughout the body. The figure shows some examples of important organs that are paced by the internal clock. These organs include the regulation of the circulatory, metabolic, endocrine and immune systems (16).
The central clock is located in the hypothalamus in the brain and works as the leader of the clock system. It receives direct information about, for example, the brightness of the environment through the ganglion cells of the eyes. Studies show the important role of the central clock: in animal experiments, after suprachiasmatic nucleus removal, animals slept and were active around the clock in a completely random manner, regardless of light or other environmental cues (17). In addition to regulating the central clock, every tissue and most cells in the body contain clock genes that activate in line with the circadian rhythm. Together with the central clock, these regulate the activity of all organs and tissues in the body according to the internal clock rhythm (18). The figure below further clarifies the function of this system.
Figure 4. Simplified illustration of the internal clock. If the cues are correctly timed according to the time of day and the rhythm remains consistent on a daily basis, the internal clock will adjust physiological and psychological functioning to optimum. Various negative consequences arise if this biological rhythm is disturbed (adapted from the source (19).
What does disruption of rhythm mean in practice?
When the rhythm is disturbed, the tissues of the body (for example, the heart, liver and kidneys) are not internally in synchrony with each other, so their function is suboptimally timed (20). This causes physiological disturbances in the body's functioning, such as changes in blood pressure, metabolic function or natural hormonal fluctuations are not optimally timed, placing unnecessary stress on the body (21, 22).
In simple terms, a disturbance of biological circadian rhythms is a stressor for the body (23). A complete discussion of the functioning of the stress response is the subject of another article, but for the purposes of this article it's enough to understand that the triggering of the stress system leads to the secretion of various hormones responsible for the so-called human stress response. One of these main hormones is cortisol, the secretion of which increases when the stress response is triggered (24). Disruption of the circadian rhythm leads to an increased secretion of cortisol, which, if prolonged, can have various negative effects. The natural circadian fluctuation of cortisol is also lost, and cortisol is constantly elevated.
Disturbances of the circadian rhythm therefore lead to a disturbance of several body processes. The examples in the figure below show how several hormones, body temperature and cognitive functions follow a precise circadian rhythm. For example, these natural rhythms are disrupted if the body's internal circadian rhythm is not regular or aligned with the circadian rhythm of the environment.
Figure 5. Examples of our body's normal timing of melatonin (25), growth hormone (26), body temperature (25, 26), cortisol (25), testosterone (27) and accuracy of mathematical processing (28, 29) for a person with a regular sleep time of 22:00 to 06:00. We have used the image with the permission of top researcher Russell Foster, taken from his book Life Time.
Summary
Ensuring a natural and regular circadian rhythm for the body is important - failure to do so will have direct physiological and psychological consequences. Optimizing the circadian rhythm, on the other hand, contributes to the optimal functioning of the body on several levels. This includes improved decision-making, mood, hormonal and metabolic function, mental and physical performance and recovery. In the following articles we will focus on how to optimise the rhythms of the internal clock and build an optimal circadian rhythm through light exposure, exercise, nutrition and sleep.
- M.Sc Exercise Physiology, Julius Granlund & Tapio Tulenheimo
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