Chronotype, the technical term for an individual’s innate preference for morning or evening activity, is governed by the circadian system, the internal biological clock that orchestrates fluctuations in hormone secretion, core body temperature, alertness, and sleep propensity across the roughly twenty-four-hour day. Most people fall somewhere on a continuum between extreme morningness and extreme eveningness, and this preference is partly genetic, remarkably stable across adulthood, and measurable with validated self-report instruments such as the Horne and Östberg morningness-eveningness questionnaire, the very tool used in the new research. What makes chronotype clinically interesting is its repeated association with mental health: large population studies have linked evening orientation to elevated rates of depression and anxiety, and a growing body of work implicates circadian misalignment in the onset and persistence of psychiatric illness.

Veterans represent a population in which these questions carry unusual urgency. Military service imposes chronic circadian disruption through shift work, deployments, night operations, and irregular schedules, and veterans are known to experience more severe sleep difficulties than community-dwelling civilians who never served. At the same time, depression, post-traumatic stress disorder, and suicidality are markedly elevated in this group, and the Department of Veterans Affairs continues to track suicide prevention as a central clinical priority. Yet, as Ulmer and her colleagues note, relatively few studies have systematically examined chronotype in veterans, leaving a gap in understanding how biological sleep timing interacts with the psychological aftermath of military service.

To address that gap, the research team drew on a multi-site post-deployment health repository, the Post-Deployment Mental Health study and registry, which enrolls US veterans who served in the Afghanistan and Iraq eras. From this registry the investigators analyzed data from 144 veterans who had completed a battery of well-validated self-report measures. Chronotype was classified using the Horne and Östberg questionnaire, a classic instrument that asks respondents about their preferred wake times, bedtimes, and periods of peak alertness. Depression symptoms were assessed with the PHQ-8, post-traumatic stress symptoms with the PTSD Checklist for DSM-5, insomnia severity with the Insomnia Severity Index, general sleep quality with the Pittsburgh Sleep Quality Index, and trauma-associated sleep disturbance with the Pittsburgh Sleep Quality Index Addendum for PTSD, a scale designed to capture nightmares and nocturnal behaviors specific to trauma survivors.

The analytical approach was deliberately conservative. The researchers used analysis of covariance to compare symptom levels across chronotype groups while statistically adjusting for age and sex, two demographic factors that can independently influence both sleep timing and mental health. They also applied false discovery rate control, a modern statistical correction that guards against spurious findings when many comparisons are tested simultaneously, a rigor that strengthens confidence in the patterns that survived. This matters because chronotype research, like much of sleep science, has historically wrestled with small samples and inconsistent methodology, and the field has called for studies that can distinguish genuine associations from statistical noise.

The results were clear and, in several respects, sobering. Consistent with prior research in civilian and military samples, evening types endorsed substantially greater symptomatology across measures of depression and PTSD compared with morning types. The night owls in the sample also reported more severe insomnia symptoms and poorer overall sleep quality. Crucially, the team tested whether these relationships could be explained simply by veterans sleeping fewer hours, and they were not: the associations between evening chronotype and worse mental health and sleep outcomes held even after accounting for sleep duration. In other words, it is not merely that late-timing veterans sleep less; something about the timing and alignment of their biological rhythms itself appears tied to their symptom burden.

One finding, however, broke from expectation. Chronotype was not associated with trauma-associated sleep disturbance as measured by the PTSD-specific addendum scale. This dissociation is scientifically intriguing. It suggests that the nightmares, night sweats, and nocturnal hypervigilance that characterize trauma-related sleep disruption may follow a different pathophysiological logic than general insomnia and poor sleep quality, one less dependent on the underlying circadian preference. The authors also point to a broader literature, including a twin study suggesting quasi-causal associations between chronotype and PTSD symptoms and an individual participant data meta-analysis linking sleep timing to post-traumatic stress, indicating that the chronotype-PTSD relationship remains an active and contested area of investigation.

Why might eveningness carry this psychiatric burden? Several mechanisms have been proposed in the wider literature. Living against the grain of one’s internal clock, a phenomenon researchers call social jetlag, forces evening types to wake for work, school, or family obligations before their biology is ready, producing chronic circadian misalignment that has been linked to inflammation, metabolic dysregulation, and impaired emotional brain circuits. Evening types also face greater exposure to darkness and social isolation during their peak waking hours, potentially compounding low mood. Disrupted circadian rhythms have been shown in translational studies to affect neurotransmitter systems, stress hormone regulation, and the architecture of REM sleep, the sleep stage most implicated in emotional memory processing and trauma-related nightmares. None of these mechanisms was directly tested in the veteran study, but they offer a plausible framework for why a late body clock and heavy psychiatric symptoms travel together.

The clinical implications are where the study’s authors place their strongest emphasis. They argue that consideration of chronotype may be important for case conceptualization in military veterans, particularly those presenting with elevated depression, suicidality, or trauma-related symptoms. In practical terms, a clinician who knows that a veteran is a pronounced evening type gains a piece of risk information that costs nothing to collect and may help identify phenotypes at increased risk of greater psychiatric symptom burden. The finding also speaks directly to treatment. Cognitive behavioral therapy for insomnia, the first-line, guideline-recommended treatment for chronic insomnia, includes components that deliberately shift sleep timing, and prior research has shown that shifts toward morningness during such interventions are associated with improvements in depression, positive affect, and sleep quality. Chronotype has also been found to moderate outcomes in digital and group-based CBT-I programs, meaning that a veteran’s circadian preference could influence how well standard sleep treatments work for them.

The study’s limitations temper but do not diminish its message. With 144 participants, the sample is modest, and the cross-sectional design means the data cannot establish whether eveningness predisposes veterans to worse symptoms, whether psychiatric illness pushes people toward later schedules, or whether both are driven by shared underlying factors. Self-reported measures, however well validated, capture perception rather than objective circadian physiology, and the registry sample may not represent all veteran populations. Still, the convergence of this veteran data with a large international literature on chronotype and mental health makes a compelling case that the question deserves a permanent place in clinical assessment. For the millions of veterans navigating the invisible injuries of service, the humble body clock, long treated as a matter of personal habit, is emerging as a measurable biological variable with real consequences for mental health, and one that clinicians can begin acting on today.

Subject of Research: Associations between chronotype and symptoms of depression, PTSD, and sleep disturbances in US military veterans

Article Title: An examination of chronotype associations with symptoms of depression, PTSD, and sleep disturbances in US military veterans

Article References: Ulmer, C. S., Wilson, S. C., Ysidron, D., & Ferguson, S. (2026). An examination of chronotype associations with symptoms of depression, PTSD, and sleep disturbances in US military veterans. Journal of Clinical Sleep Medicine, 22(1), Article 183. https://doi.org/10.1007/s44470-026-00165-w

Image Credits: AI Generated

DOI: 10.1007/s44470-026-00165-w

Keywords: chronotype, military veterans, depression, PTSD, insomnia, sleep quality, circadian rhythms, eveningness, cognitive behavioral therapy for insomnia, sleep timing, mental health, post-deployment health