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A growing body of research suggests a bidirectional effect of circadian rhythm on metabolic and mental health. A cross-sectional 2023 analysis published in Nature of over 80,000 adults in the UK Biobank, revealed that increased exposure to artificial light at night— known to disrupt circadian rhythm– was associated with increased rates of MDD, bipolar disorder, GAD and PTSD.
Circadian rhythm dysfunction has interactions with nearly all metabolic processes and has been shown to induce cardiometabolic and inflammatory disturbances—similar phenotypes observed in those with depression. Sunlight serves as the primary synchronizing input to our circadian clock, promoting alignment within the body’s systems. Biologically we are programmed to obtain light from the sun, as it supports balance of our internal circadian rhythm—significant for homeostasis of integral processes that govern our health.
Among a number of factors, exposure to artificial light at night suppresses melatonin—an essential hormone for circadian synchronization—impeding this process. Several lines of evidence support this. A 2023 review published in The International Journal of Molecular Sciences demonstrated connections between misaligned circadian rhythm and overall physiological dysregulation. Significant disruptions were observed in metabolism and hormonal balance in those exposed to artificial light at night and engaged in shift work.
Recent research published in The Annals of Nutrition and Metabolism revealed that long-term circadian disruptions are associated with a number of metabolic conditions including obesity, impaired glucose tolerance, diabetes, in addition to psychiatric disorders.
A further study published in The Journal of Biological Rhythms examined the metabolic and physiological consequences of circadian rhythm disruption in rats by exposing them to 24 hours of constant light. Rats prone to diabetes exhibited early diabetes onset, attributed to accelerated apoptosis and loss of beta-cell function.
A bidirectional relationship seems to exist between mood disorders and circadian rhythm, as demonstrated in a 2020 review published in Translational Psychiatry. Altered circadian rhythm was reported amongst those with bipolar disorder, anxiety, schizophrenia & MDD based on correlational clinical data. Experimentally induced disruptions of circadian rhythm on healthy rats also revealed changes in affect, further supporting these findings. Severity of symptoms may be correlated with the extent of circadian rhythm misalignment, as exhibited in this study of individuals with MDD.
Several neural pathways impacted by artificial light at night have been found to contribute to depressive and mood disorders. Sunlight exposure, on the other hand, regulates mood via ipRGC projections to the perihabenular nucleus (PHb) circuit, as evidenced in this 2018 study published in Cell.
The circadian clock comprises the central clock, part of the molecular system—located in the suprachiasmatic nucleus (SCN) of the hypothalamus—where approximately 20,000 neurons exist and receive light cues. It also consists of peripheral clocks that are widely distributed throughout the body. Information from this “master clock” is conveyed to the rest of the body via humoral and neural signaling, with expression in the intestines, liver, kidneys, lungs, and heart.
This system controls the expression of various genes and is a regulatory mechanism for far-reaching aspects of health. Disruptions in daily rhythm have been shown to affect neurotransmitter and hormone levels, as well as the sleep-wake cycle.
A further 2024 study explored how screen time impacts sleep quality in a cohort of 1000 school-aged children. Subjects were divided into two categories based on their daily screen exposure. A low screen time group was established as less than one hour of screen time daily, whereas a high screen time group was defined as over three hours of daily screen time. The analysis revealed that children with low screen time had better sleep efficiency (90% vs. 75%) than those with high screen time. They also had reduced risk of circadian rhythm disturbances, whereas those in the high screen time group had a 20% increased risk in disturbance. This study further underscores the sensitivity of this system to misalignment.
A longitudinal 2015 study published in Sleep Medicine of day shift workers who transitioned to rotating shift work schedules, reported higher levels of anxiety along with disordered sleep. Individuals who developed Shift Work Disorder (SWD) reported greater increases in symptoms of both depression and anxiety. Nurses with SWD were also found to have elevated anxiety scores, measured by the Hospital Anxiety and Depression Scale. One third of nurses exhibited symptoms indicative of SWD, with the highest prevalence in schedules involving night shifts.
Subsequent studies have explored how disrupted circadian rhythm contributes to the development of anxiety-like behavior, as evidenced in a study published in The Journal of Behavioural Brain Research that found exposure to artificial lighting for 8 weeks induced increased anxiety behavioral responses in rats.
Alterations in gut microbiota have also been exhibited in those with impeded circadian rhythm, as evidenced in this 2023 review, supporting evidence of bidirectional communication between the microbiome and circadian rhythm disruption.
Similar to circadian rhythm, the gut microbiome is also a key regulator of health, with notable effects on host health. Stress and gut microbiota may be mediating factors in circadian rhythm, as well as in mood and metabolic health. Meal timing, nutrition, appropriate light exposure, sleep scheduling, and exercise can help in supporting these processes.
A 2019 review highlighted the role stress induces in the system, and how this impedes circadian rhythm and the serotonin system, while conferring an increased risk to depression. Findings of a 2023 review study highlight the interplay between circadian rhythm, the microbiome and the epigenome, underscoring the pivotal role nutrition has in influencing these processes. A study published in Cell discovered that the integrity of the intestinal epithelial cells (IEC) circadian clock is warranted for microbiota and IEC symbiosis and signaling—integral for rhythmic regulations of nutrient absorption, gut motility and metabolism. Modalities that target resynchronization of circadian rhythm may be a promising approach for improving metabolic and mental health conditions.
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