Chronic Circadian Misalignment in Military Personnel: A Potential Contributor to Neuroinflammation Risk

Chronic circadian misalignment has long been treated as an unfortunate byproduct of modern life, a nuisance to be managed rather than a biological insult in its own right. A new perspective article published in Frontiers in Molecular Neuroscience argues that this framing is fundamentally wrong. Drawing on evidence from experimental models, civilian studies, and a growing body of military research, the authors contend that persistent disruption of the body’s internal clock should be reconceptualized as a potential, modifiable contributor to neuroinflammatory risk, capable of driving brain dysfunction even in otherwise healthy and physically fit populations. Military personnel, they argue, represent a unique and underutilized human model for testing this idea, because their careers expose them to predictable, repeated circadian stressors that few civilian occupations can match.

The circadian rhythm is an endogenous timing system that regulates physiological and behavioral processes over an approximately 24-hour cycle. In mammals, the central pacemaker resides in the suprachiasmatic nucleus of the anterior hypothalamus, which receives direct photic input from intrinsically photosensitive retinal ganglion cells through the retinohypothalamic tract. Light serves as the primary environmental cue, synchronizing the endogenous clock with the external day-night cycle. At the molecular level, circadian rhythms are generated by a transcriptional-translational feedback loop involving core clock genes, including CLOCK, BMAL1, PER1-3, and CRY1-2. These genes regulate the rhythmic expression of thousands of downstream genes across multiple tissues, allowing biological processes to occur at the most appropriate time of day. In addition to the central pacemaker, peripheral clocks are present in nearly every organ, including the liver, heart, skeletal muscle, adipose tissue, and brain, coordinating local physiological functions while remaining synchronized with the suprachiasmatic nucleus.

The authors are careful to distinguish circadian misalignment from related but mechanistically distinct exposures. Circadian misalignment refers specifically to a mismatch between the endogenous circadian phase, as reflected in core body temperature, melatonin, or cortisol rhythms, and the imposed sleep-wake or work schedule, and is properly demonstrated only when circadian phase markers are directly assessed. Sleep restriction or deprivation refers to a reduction in total sleep time or complete sleep loss, which can occur with or without a shift in circadian phase and can independently produce inflammatory and cognitive effects. Night work, shift work, and irregular scheduling describe occupational exposures that frequently, but do not necessarily, produce circadian misalignment. Operational stress refers to psychological or physiological strain associated with military duties, which may co-occur with but is mechanistically distinct from circadian disruption. Altered light exposure is a zeitgeber-level exposure that can shift circadian phase but is not equivalent to misalignment itself.

At the molecular level, the proposed pathway from disrupted clocks to inflamed brains runs through mitochondria. Circadian misalignment disrupts core clock gene regulation, leading to downstream alterations in mitochondrial function and redox homeostasis. Mitochondria play a central role in maintaining neuronal homeostasis by regulating adenosine triphosphate production, calcium signaling, redox balance, and innate immune responses. When mitochondrial function falters, the result is excessive production of mitochondrial reactive oxygen species, impaired oxidative phosphorylation, and the release of mitochondrial damage-associated molecular patterns, which activate microglia and promote neuroinflammatory signaling. Mitochondrial reactive oxygen species can also stimulate NLRP3 inflammasome activation and NF-κB signaling, thereby increasing the production of pro-inflammatory cytokines, including interleukin-1β, interleukin-6, and tumor necrosis factor alpha. Consequently, mitochondrial dysfunction and oxidative stress are increasingly recognized as key drivers of chronic neuroinflammation and the associated cognitive and affective impairments.

A second critical interface is the blood-brain barrier, the highly specialized neurovascular interface that maintains central nervous system homeostasis by regulating the transport of molecules between the circulation and brain tissue. Disruption of barrier integrity increases vascular permeability and facilitates infiltration of peripheral immune mediators, including pro-inflammatory cytokines, thereby promoting microglial activation and neuroinflammatory responses. Importantly, preliminary evidence from military populations provides some support for an association between severe operational stress and altered barrier permeability. In soldiers undergoing intensive combat training, circulating levels of S100B, a marker commonly used to indicate altered blood-brain barrier permeability, were increased. Similarly, a study of U.S. Marines undergoing Survival, Evasion, Resistance, and Escape training reported changes in S100B alongside alterations in inflammatory and stress-related biomarkers. However, these studies did not specifically assess circadian misalignment, and therefore do not establish a direct causal relationship between circadian disruption and barrier dysfunction. Rather, they provide military-specific human evidence supporting barrier alterations as a potential component of the broader physiological response to severe operational stress.

What makes military personnel such a valuable model is the sheer density and predictability of their circadian stressors. Unlike many civilian populations, military service routinely exposes individuals to multiple, predictable sources of circadian disruption, including shift work, prolonged wakefulness, night operations, transmeridian travel, operational stress, and irregular sleep schedules. These stressors often occur simultaneously and repeatedly throughout military careers, creating sustained circadian misalignment despite otherwise high levels of physical fitness and medical screening. Emerging evidence suggests that chronic circadian disruption may compromise operational performance by affecting cognitive flexibility, executive function, decision-making, emotional regulation, and stress resilience. These functional impairments are increasingly hypothesized to reflect molecular mechanisms involving neuroinflammation rather than sleep loss alone, although the authors acknowledge that this link is a translational inference drawn mainly from preclinical and experimental evidence, as direct evidence of neuroinflammation in operationally impaired military personnel is lacking.

The evidence across military domains is revealing, if fragmented. A recent study in military cadets demonstrated that total sleep deprivation disrupted circadian rhythmicity, reduced cognitive performance, and altered mood, with nocturnal light exposure transiently improving alertness while simultaneously perturbing circadian rhythmicity. In U.S. Air Force T-6A Texan II instructor pilots, increased serum levels of proinflammatory cytokines, including interleukin-6 and tumor necrosis factor alpha, were associated with elevated fatigue during operational flight duties. A controlled study involving active-duty military personnel found that perception-action coupling performance was significantly influenced by both time of day and simulated military operational stress, with performance deteriorating during circadian low points and under operational stress, and the combined effects exceeding those of either factor alone. A recent meta-analysis of human experimental studies found that several consecutive nights of partial sleep deprivation, rather than circadian phase misalignment, which was not assessed, significantly increased circulating interleukin-6 and C-reactive protein concentrations, suggesting that sustained sleep loss alone can induce a systemic inflammatory response that may contribute to neuroinflammation and subsequent cognitive and emotional dysfunction via humoral and neural immune-to-brain signaling pathways.

Aviation and maritime operations add further layers of complexity. Military fighter pilots are routinely exposed to a combination of distinct circadian-relevant stressors, including circadian phase misalignment, irregular work schedules, sleep restriction, and high operational demands, although these exposures are rarely disentangled in the available literature. A recent nested case-control study in Danish military personnel reported that pilots had a higher incidence of prostate cancer than non-flying military controls, with the authors proposing that repeated circadian disruption associated with transmeridian flights, together with occupational exposures such as ionizing radiation and prolonged sedentary work, may contribute to this increased risk. Submarine deployment represents a unique military environment characterized by several co-occurring but mechanistically distinct exposures: circadian phase misalignment, sleep loss, and the absence of natural light cues. A submarine study found that the 6-hour-on/6-hour-off schedule led to fragmented, biphasic sleep during a 67-day undersea mission, with melatonin failing to adapt to the imposed work schedule while cortisol showed greater adaptation, demonstrating that human physiology cannot fully adapt to operational demands. In submariners, combined stressors were associated with alterations in gut microbiota composition and mood, with microbiome changes correlating with fatigue, although because these exposures were not disentangled, the observed effects cannot be attributed specifically to circadian misalignment rather than to sleep loss or light deprivation alone.

The authors also emphasize the principal evidence gap for the field. Direct military studies that combine objective circadian biomarkers, such as melatonin, cortisol, and actigraphy-derived phase markers, with inflammatory or neuroimmune biomarkers and neurocognitive or affective outcomes within the same cohort remain scarce. Much of the available literature is observational or based on short-term experimental studies, limiting conclusions regarding causality and long-term neurological consequences. Moreover, circadian disruption frequently co-occurs with sleep restriction, shift work, and operational stress, making it difficult to disentangle their independent effects. Inflammatory evidence is also predominantly based on peripheral biomarkers, which provide biological plausibility but do not directly demonstrate central neuroinflammation. Military populations are heterogeneous with respect to occupational demands, deployment conditions, schedules, and environmental exposures, which may contribute to variability across studies. These limitations highlight the need for well-controlled, longitudinal studies integrating objective circadian measures with peripheral and, where feasible, central neuroimmune markers and functional outcomes.

Despite these caveats, the reframing carries significant implications for both research and practice, and the authors outline a suite of potential countermeasures grounded in existing evidence. Exposure to bright light during nighttime military operations has been shown to improve subjective alertness, reduce sleepiness, and help maintain performance, while a study of 42 active-duty U.S. Navy submariners demonstrated that individualized light therapy improved sleep quality, enhanced projected performance, and facilitated circadian adaptation during a 12-day operational deployment. Strategic napping offers another practical approach, with a 30-minute nap shown to mitigate performance deterioration following sustained military operations involving partial sleep deprivation. Controlled experimental evidence further demonstrates a dose-dependent decline in sustained attention, reaction time, and cognitive performance with progressively restricted sleep duration, supporting the value of protected sleep opportunities, stable sleep windows, and optimized duty-rest schedules. Individual differences in circadian preference should also be considered, as chronotype can influence adaptation to shift work, and assessment using validated tools such as the Morningness-Eveningness Questionnaire may support more individualized approaches to managing circadian strain. Finally, a randomized trial among Royal Australian Navy sailors demonstrated that a brief educational program focusing on circadian rhythms, strategic light exposure, and sleep hygiene improved sleep quality and psychological health during at-sea deployments. Together, these findings suggest that circadian misalignment in military personnel is not merely a byproduct of performance demands, but a biologically plausible contributor to brain dysfunction that warrants targeted intervention and further mechanistic study, with the potential to protect both short-term operational readiness and long-term neurological health.

Subject of Research: Neuroscience

Article Title: Chronic circadian misalignment in military personnel: a potential contributor to neuroinflammation risk

Article References: Novak, A., & Teglas, T. (2026). Chronic circadian misalignment in military personnel: a potential contributor to neuroinflammation risk. Frontiers in Molecular Neuroscience, 19, Article 1960208. https://doi.org/10.3389/fnmol.2026.1960208

Image Credits: AI Generated

DOI: 10.3389/fnmol.2026.1960208

Keywords: circadian rhythm, military, neuroinflammation, blood-brain barrier, mitochondria, sleep deprivation, cognitive dysfunction, mood disturbances, Frontiers in Molecular Neuroscience, Chronic, circadian, misalignment