How gut bacteria may shape your sleep

Photo: Jp Valery on Unsplash

A comprehensive review examines current insights into the link between microbial imbalance in the gut and various sleep disorders.

In a large-scale review published on 4 November in Brain Medicine, a Chinese research team brings together current knowledge on the complex relationship between gut bacteria and sleep. ‘The authors have compiled a great many studies into a solid and highly readable overview, but it does not yield genuinely new insights’, says Benno Haarman, researcher and psychiatrist at University Medical Centre Groningen.

The human gut harbours trillions of micro-organisms that communicate with the central nervous system via multiple pathways. ‘We already know from several studies that people with sleep disorders often have a different composition of gut bacteria, and that certain bacterial compounds – such as short-chain fatty acids and bile acids – can influence sleep and mood’, says Haarman. The review shows that patients with chronic insomnia, for example, display reduced microbial diversity and an aberrant abundance of specific bacterial families compared with control groups. Similar patterns are seen in sleep apnoea, where a lower concentration of beneficial gut bacteria correlates with the severity of the condition. According to multiple studies, short-chain fatty acids – compounds produced by bacterial fermentation of dietary fibre – offer protective effects against sleep disruption. Elevated concentrations of bile acids, another class of microbial metabolites, are by contrast associated with chronic insomnia.

Vicious cycles
These are, one and all, associations that scientists had already reported previously, according to Haarman, but the direction of the effect remains unknown. The review authors point out that imbalance of the gut microbiome may be both a consequence of and a contributor to sleep disturbances, potentially creating vicious cycles that perpetuate health problems and sleep deprivation. ‘It looks more like a vicious cycle than a straightforward cause-and-effect relationship’, Haarman agrees. He adds: ‘Causal evidence in humans is still limited. A few small studies suggest that taking live beneficial bacteria or bacterial compounds can modestly improve sleep quality, but the effects are small and not the same for every individual.’

Therapeutic strategies aimed at restoring microbial balance nevertheless show promising results, according to the review. Although no studies have yet successfully compared different microbiota-targeted therapies with one another, the existing evidence suggests that each approach – from probiotic supplementation to more invasive faecal microbiota transplantation – offers specific benefits for improving sleep quality. Whilst the authors stress that more comparative research into such treatments is needed, they argue that it holds promise for personalised treatment of sleep disorders. The review concludes with a systematic framework for future research, with clinical applications as the ultimate goal. Haarman considers the call for further research well-founded. In his view, the next step lies in well-controlled studies in which researchers follow individuals repeatedly over an extended period. ‘That can reveal what is cause and what is effect. Only when we truly understand the direction of the effects can we begin to think about microbiome-targeted treatments for sleep disorders.’