The Role of Interactions along the Brain–Gut–Microbiome Axis in the Regulation of Circadian Rhythms, Sleep Mechanisms, and Their Disorders

The bidirectional communication between brain structures and the gastrointestinal tract involving the microbiota defines the scientific concept of the brain–gut–microbiome axis. The intestinal microbiome plays an important role in many physiological and biochemical processes in the body, in the immune response, in the maintenance of homeostasis, and in the regulation of circadian rhythms. There is a relationship between higher prevalences of a number of neurological disorders, sleep disorders, and changes in the intestinal microbiota, so it is important to study the complex mechanisms of such correlations for the development of new treatment and prevention strategies. Environmental factors associated with excessive exposure to light can aggravate dysbiosis of the intestinal microflora, producing sleep disorders as a result. This review discusses the integrative mechanisms of regulation of sleep involving the intestinal microbiota (the roles of neurotransmitters, short-chain fatty acids, unconjugated bile acids, bacterial cell wall components, and cytokines). Taking account of the influences of intestinal dysbiosis as a risk factor in the development of various diseases, this review systematizes the key aspects and modern scientific data on the importance of the balance of the microflora to ensure optimal interactions along the brain–gut–microbiome axis in the context of the regulatory role of the sleep-waking cycle and its disorders.