A core element of digestive health, the gut microbiome is the ecosystem of microbes (and their DNA) in the gastrointestinal tract. Containing trillions of bacteria in compositions unique to each individual, the gut microbiome is associated with physiological processes in the body, including the sleep/wake cycle, an element of circadian rhythm.

Exploring the connection between the communities of bacteria in the gut and their relationship to circadian rhythm is key to understanding how the gut microbiome may influence host sleep health. In the first of our two-part blog uncovering this relationship and what it may mean for our future understanding of sleep health, discover more about the body’s natural clock and where gut health fits into the equation.

What is circadian rhythm?

The circadian 24-hour timing system (or circadian ‘rhythm’) exists in all kingdoms of life and is linked to our sleep-wake cycle. The 24-hour light/dark cycle is the stimulus that organisms adapt their own timing systems to, from cellular timing to physiological behaviours1. Sometimes referred to as the ‘body clock’, circadian rhythm helps to maintain homeostasis and works by daily changes in Clock gene expression, along with responses to external stimuli. Humans are diurnal, meaning daily or active during the day, with a rest phase at night and are therefore at risk of circadian rhythm and sleep disruption1.

The circadian rhythm is important for the maintenance of many biological processes, including the sleep-wake cycle, immune function, hormone secretion, body temperature, glucose homeostasis and intestinal function, amongst others. With the influence that circadian rhythm has on significant biological processes, it’s no wonder that disruption to the circadian rhythm can have health consequences2.

The internal timing of biological processes is influenced by the circadian rhythm, and these daily processes are controlled by the suprachiasmic nucleus (SCN) in the anterior hypothalamus of the brain. The SCN receives information from the external environment by light entering the brain through the eyes.

There are non-light time cues that act on or influence the circadian rhythm, such as food, exercise and caffeine. Irregular life rhythms can also impact the circadian rhythm, such as shift working that disrupts our diurnal patterns, or jet lag as a consequence of flying2. Relative daylight deprivation is more common in winter, in workplaces where people do not have direct access to natural light or in the case where vulnerable people do not have regular access to natural light. Changing light patterns and lack of access to natural daylight impacts mood and sleep. It is evident from seasonal research in the Antarctic that there is more melatonin suppression as a result of reduced daylight during the winter. This suppression of this sleep hormone is due to increased retinal sensitivity to light and could be a reason for sleep disruption3.

Daylight deprivation in winter is an example of an external factor that can affect timing or duration of sleep due to the circadian rhythm disruption. This does not just refer to an unconventional sleeping pattern; circadian rhythm disorders or disruption mean a symptomatic burden based on the changes to sleep timing and quality. Shift work and jet lag can contribute to circadian rhythm changes, as well as seasonal effects from lack of light exposure4.

Gut microbiome, sleep and circadian cycles

A lack of sleep, circadian rhythm, food intake, mood, and the gut microbiome have complex relationships. Circadian rhythm disorders and insomnia are linked to metabolic syndrome and other inflammatory diseases. It is suggested that chronic disruption to normal sleep, diet patterns and the environmental clock leads to obesity5.

Fundamentally, circadian rhythm disorders change the balance of the gut microbiome and an increase in pro-inflammatory gut bacteria are associated with metabolic syndrome. Additionally, the change in sleep time can lead to physiological stress, which also alters the host’s normal gut microbiome. The association between circadian rhythm, the gut microbiota and inflammatory responses clearly demonstrates how disruptions to both can compromise the immune system5.

We know that poor sleep quality or quantity leads to metabolic, immunological and cognitive deficits. A significant increase in the Firmicutes: Bacteroidetes (two of the most prevalent phyla in the gut microbiome) ratio has been shown with 5 days of sleep disruption. This demonstrates a community shift in the gut microbiome, similar to that seen in obesity. Notably, there was a significant decrease in the Bifidobacterium genus6. After five days of disrupted sleep, a clear reduction in beneficial bacteria is seen. Additionally, altered metabolic function of the gut microbiome can be seen for at least four days after the disrupted period of sleep. There were also changes to the faecal levels of metabolic end products that are modified by bacteria. The decline in beneficial bacteria and change in metabolic function of the gut microbiome, suggests mechanisms by which disrupted sleep can cause physiological changes6.

The mechanisms by which the gut microbiota is linked to circadian rhythm are still not fully understood and the wider picture and implications for clients and patients is yet to be uncovered, but there are clear interactions demonstrated between the circadian Clock gene and the gut microbiome bacteria7.

A mutation of the Clock gene has been associated with intestinal barrier dysfunction. In preclinical trials, it is shown that in subjects with a mutation of the Clock gene, there is an overlap with features related to environmental circadian rhythm disruption. It may be that circadian rhythm disruption drives the dysbiosis that leads to inflammation7.

Turning knowledge into action, to what extent could our gut microbiome be altered to promote better sleep health through its role and actions?

Click here to read part two of our blog.

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