Over time, chronically inadequate sleep can actually increase a person’s risk of anxiety, depression and other mental health disorders (1). Sleep also impacts cellular growth and repair, stress hormones, our immunity, appetite, breathing, blood pressure and cardiovascular health - to name a few (2). Chronic lack of sleep can increase your risk of heart disease, high blood pressure, obesity, and stroke. More acutely, a lack of sleep can weaken your immunity and leave you more vulnerable to common infections such as the cold or flu (3).

Despite its fundamental role in promoting health, for many people falling and staying asleep can be especially challenging. Sleep disorders are becoming increasingly prominent in today’s society, with insomnia being one of the most common of these disorders. A study published in Sleep Medicine in 2021 found that, from a survey of over 22,000 people around the world, one in three participants had clinical insomnia symptoms and nearly 1 in 5 met the criteria for insomnia disorder (an inability to fall or stay asleep for at least 3 nights per week for more than 3 months), both of which can be highly disruptive to everyday life (4). For others, it is simply not prioritised. Researchers call this willful sleep deprivation (5), and there are many reasons why people put off sleep, such as longer work hours, distractions from phones and other devices and the feeling that late nights are the only time they have for ‘downtime’.

What regulates sleep?

Melatonin, produced by the pineal gland in the brain, is the main hormone that helps to initiate sleep. Its production is triggered by the setting sun and the darkness of night. Opposing melatonin is a hormone released by the adrenal gland called cortisol, which many people know of as the stress hormone. The release of cortisol helps to wake us up and keep us alert throughout the day while melatonin production is suppressed. The cycling of melatonin and cortisol with night and day regulates our internal clock and our sleep-wake cycle, which is known as the circadian rhythm. Multiple areas of the brain involving various neurotransmitters also work together to control our sleep-wake cycle, with some neurotransmitters working with cortisol to promote wakefulness and others working with melatonin to promote sleep. Sleep is also influenced by a wide range of signals coming from all over the body, including the gut (5).

How much sleep should I be getting?

It is generally recommended that adults get around 7-9 hours of sleep on a regular basis to promote optimal health and functioning. However, just as important as the number of hours of sleep is the quality of the sleep that we’re getting (1). A good night’s sleep consists of 4 to 5 sleep cycles. Each cycle includes periods of deep sleep and rapid eye movement (REM) sleep, with the amount of REM sleep increasing with each progressive cycle. This pattern of cycling and progression is critical to the biology of sleep (2).

While there are many factors which can disrupt sleep and reduce sleep quality, the influence of the gut microbiota on the body’s own internal clock and sleep cycles is an emerging area of research with some findings that may make us rethink the importance of our gut health and the community of microorganisms that it houses (5).

The gut microbiome

Advancements in technology have revealed that the community of microorganisms living in our gut, known as the gut microbiome, can impact our health and influence the function of many of our vital organs - including the brain, which is the control centre of sleep. When the gut microbiome is healthy, the whole body benefits. However, the gut microbiome can become disrupted and negatively impact our health. This is referred to as ‘gut dysbiosis’ and it has now been associated with many chronic diseases, such as obesity, asthma and osteoporosis, as well as a number of mental health and neurodegenerative disorders, including depression, Alzheimer’s disease and Parkinson’s disease (5).

The ability for the gut microbiota to impact neuropsychiatric conditions is explained by the concept of the ‘gut-brain axis’, and this link points towards the gut microbiota’s potential impact on mental wellbeing, sleep quality and a problem more and more of us are facing: insomnia.

The gut-brain axis

The ‘gut-brain axis’ refers to the two-way communication channels that exist between your gut and your brain. Many different body systems allow the gut and brain to communicate. For example, stress hormones released by the brain can alter gut function and change our bowel habits, and a major nerve (called the vagus nerve) carries an extensive range of signals from the digestive system to the brain and vice versa. These are just some examples of the complex metabolic, immunological, neurochemical and hormonal channels of communication between the gut and the brain (6).

The messages being sent to the brain from the gut can impact brain activity and alter our behaviour and emotions. The living community of microorganisms in the gut also communicate with the brain via these channels, and they have the ability to change the types of messages being sent from the gut to the brain. In this way, the health of our gut and our gut microbiota may be influencing the risk of poor mental health and insomnia (6).

The impact of the gut microbiota on brain activity

Our understanding of the impact of the gut microbiota on brain activity and mental health has been deepened by a number of key animal studies. Prior to these studies, disruptions to the gut microbiota and poor mental health had been observed together but researchers had not yet shown that the disturbed gut microbiota was contributing to these psychological changes (7).

In 2011, researchers at McMaster University were able to make calm mice anxious and anxious mice calm by performing faecal microbiota transplantations (i.e. transferring their faeces) between the two groups (7). By simply swapping their gut microorganisms and nothing else, the researchers were able to completely change the behaviour of the mice. Likewise, another landmark study in 2016 demonstrated that transferring the microbiota of depressed human patients to mice caused them to behave in a depressed manner (8). Together, these studies demonstrate that changes in the gut microbiota can directly alter behaviour and mental wellbeing via the gut-brain axis.

The relationship between the gut microbiota and sleep

Many different microorganisms live within the gut, each playing an important role and working together to create the conditions for a healthy gut and body. An imbalance of particular microorganisms within the gut is known as gut dysbiosis, and it can be disruptive to many aspects of our health - including sleep.

Gut dysbiosis has been shown to alter the body’s circadian rhythm, changing the expression of genes which control the release of cortisol and other molecules our body needs to accurately perceive the time and behave accordingly. Specifically, gut dysbiosis has been shown to increase the release of cortisol. In this way, the gut microbiota has the potential to disrupt the balance between cortisol and melatonin that is so important for the sleep-wake cycle (9).

There are many different mechanisms by which the gut microbiota could influence sleep, but their production of neurotransmitters has garnered a lot of attention. Scientists have identified a number of different bacteria within the gut microbiota which can produce neurotransmitters involved in the sleep-wake cycle. For example, Bacteroides are a group of bacteria found in the gut which have been shown to produce a high amount of gamma-aminobutyric acid (GABA), a neurotransmitter well-known for promoting sleep, preventing anxiety and stress, and balancing mood. Many medications used to treat insomnia target this neurotransmitter and the receptors that it binds to in the brain in order to help promote sleep. Conversely, some bacteria in the gut can produce histamine which, as well as being involved in allergies, also acts as a neurotransmitter that promotes wakefulness and mental alertness. As different groups of bacteria produce GABA and histamine, maintaining a balance between these two within the gut may help to support a healthy sleep-wake cycle. Disruptions to this microbial balance, it has been suggested, may be implicated in insomnia (5).

Studies have begun to show that a more diverse gut microbiota is associated with longer undisturbed sleep, highlighting the potential for a healthy gut microbiota to positively influence the mechanisms which regulate our sleep. Diversity is one of the best markers of a healthy gut microbiota, as many different types of microorganisms are needed to maintain a healthy gut and body. Indeed, changes in the diversity and the types of microorganisms within the gut have been observed between people with insomnia and those without, and greater reductions in microbial diversity were observed in those individuals with more chronic insomnia (5).

What causes gut dysbiosis?

The rise in gut dysbiosis and reductions in microbial diversity are thought to be due to several factors. Changing diets with lower intakes of fibre and unrefined plant foods, stressful lifestyles, low activity levels and increased use of antibiotics – these factors reduce the population and change the types of microorganisms found in the gut, creating gut dysbiosis. Interestingly, low fibre, high saturated fat and high sugar diets - three factors linked to gut dysbiosis - have also been associated with poorer quality sleep (10).

Targeting the gut-brain axis for improved sleep

The understanding of the gut-brain axis has led many researchers to investigate if supplementing with probiotics can help to improve the influence of the gut on brain activity, mental health and sleep. Probiotics are beneficial bacteria which can help to restore balance to the gut microbiota and provide additional health benefits to the human body while they are present in the digestive tract. However, not all probiotics offer the same benefits and this depends entirely on the specific strains being used.

Just like the microorganisms found naturally in your gut, specific probiotic bacteria can influence the gut-brain axis. Supplementing specific probiotic strains known to affect the gut-brain axis in favourable ways is an easy and accessible way to begin the process of improving gut-brain axis communication, which can also be further supported by diet and lifestyle interventions.

For example, in a randomised, double-blind, placebo-controlled study, 38 healthy participants supplemented the combination of probiotic strains found in Biome Lift Probiotic or placebo for 6 weeks and were assessed for changes in mood and sleep quality. Significant improvements for the participants taking the probiotic were found after 6 weeks, including a reduction in low moods and improved sleep quality and cognitive fatigue (11). This same formulation is the subject of two other ongoing clinical trials, with promising early results.

Where is gut-brain axis research headed?

While further studies are needed to shed more light on the complexities of the gut-brain axis, early research supports that a healthy and diverse gut microbiome is beneficial to gut-brain axis communication and may help to improve the quality of your sleep - as well as provide benefits to many other areas of health. Probiotics targeting the gut-brain axis are also emerging as a promising tool of the future for helping people manage poor sleep and support their mental wellbeing.