Night Shift Work Cohort Tracks Biological Wear Across a Healthcare Workforce
The EPHOR-NIGHT cohort gives occupational medicine a rare long view of night shift work, pairing detailed exposure tracking with layered biomarker profiling in workers whose schedules have already pushed sleep, recovery and circadian alignment off balance. Built as part of the broader exposome approach in health and occupational research, the study was designed to move beyond job titles and shift labels and toward the biological pathways that may connect work at night with cardiometabolic, mental health, cognitive and ageing-related outcomes.
The cohort includes 937 workers aged 20 to 65 years, and women made up 88% of participants. Most were employed in healthcare, which gives the project practical relevance for hospitals and other care settings where night work is routine rather than exceptional. Participants were grouped as permanent day workers, permanent night workers or rotating and other shift workers, creating a structure that can separate the burden of a fixed circadian misalignment from the instability of changing schedules.
A short line captures the study’s clinical value: this is not a simple survey of shift work; it is a biologic exposure map.
Exposure Measurement Extends Far Beyond a Standard Occupational Questionnaire
The study design is unusually dense for an occupational cohort. Participants completed questionnaires, daily ecological momentary assessments, wearable sensors that tracked light exposure, physical activity, heart rate and environmental conditions, and biological sampling that included blood collected once and saliva collected at five time points across the day. Harmonised protocols were used across Spain, Sweden, Denmark and the Netherlands, which matters because cross-country occupational research often falters when measurement differs from site to site.
By integrating real-time data with biospecimens, the cohort can interrogate mechanisms that are easy to miss in conventional shift work studies. Light exposure affects circadian timing; sleep disruption can alter appetite regulation and glucose handling; repeated schedule stress may affect autonomic tone and inflammatory signaling. The result is a framework that can examine not only whether night work correlates with chronic illness, but how chronic illness risk may accumulate through sleep loss, stress physiology and behavioural changes that operate together rather than in isolation.
Among the 937 participants who contributed data, 708 had complete information from questionnaires, sensors and blood and saliva. Subsets then underwent advanced laboratory analyses spanning genomics, targeted and genome-wide DNA methylation, telomere length, mitochondrial DNA copy number, metabolomics, transcriptomics, proteomics, hormone profiling, inflammatory biomarkers and blood metals. Few occupational cohorts combine this range of readouts at a single time point, and fewer still do so in a workforce where shift-related disruption is built into the daily schedule.
Sleep and Mental Health Burden Appeared Highest in Permanent Night Workers
Baseline findings already point toward a clinically meaningful burden in this workforce. Anxiety was reported by 27% of participants, depression by 18%, and metabolic disturbances were also common. Those conditions matter because they sit at the intersection of sleep debt, stress biology and downstream cardiometabolic risk. Night shift work can fragment sleep, suppress melatonin timing, alter cortisol rhythms and reduce recovery time between shifts, creating a physiologic environment in which mood symptoms and metabolic dysregulation can reinforce one another.
Sleep duration and sleep quality were poorest among permanent night shift workers. In a cohort built to measure exposure with precision, that pattern is especially important because it supports a dose-like relationship between circadian disruption and sleep impairment rather than a vague occupational association. Permanent night workers also experienced greater exposure to long shifts and had more scheduled rest days than day workers, a scheduling pattern that may reflect attempts to compensate for occupational strain while still leaving workers exposed to prolonged wakefulness and irregular recovery windows.
Permanent night work carries a blunt physiologic cost: the body is asked to behave as if daytime is nighttime, repeatedly and without full recovery.
Why the Biomarker Strategy Matters for Chronic Disease Research
The value of EPHOR-NIGHT lies in its capacity to connect exposure, intermediate biology and disease-relevant phenotypes. Sleep loss alone rarely explains the full burden seen in shift workers. Circadian misalignment can influence glucose tolerance, blood pressure regulation, systemic inflammation and lipid metabolism, while chronic stress may further modify immune and endocrine responses. The cohort’s biomarker strategy gives investigators the tools to ask whether those pathways are visible in genomics, methylation patterns, telomere dynamics, mitochondrial function or circulating proteins and metabolites.
The salivary sampling schedule across five points in the day also adds an important temporal layer. Hormones and stress-related markers are not static measures; they rise and fall across the day, and shift work can flatten or invert normal rhythmicity. Measuring these signals across time may help identify whether night work is associated with altered diurnal biology even before overt disease becomes apparent. For occupational health, that distinction could shape earlier prevention strategies rather than waiting for established diabetes, hypertension or major depressive symptoms to emerge.
Female sexual and reproductive health is also part of the follow-up plan, which is relevant because women dominate the cohort and because night shift schedules may intersect with hormonal, sleep and reproductive biology in ways that standard occupational surveillance often misses. The added biomarker collection completed at the 2-year follow-up in June 2025 expands the project’s ability to study change over time rather than relying only on baseline snapshots.
Two-Year Follow-up Expands the Cohort’s Policy Relevance
The 2-year follow-up introduces another layer of usefulness for occupational medicine and workplace policy. Additional biomarker data, psychosocial work environment measures and reproductive health information will help determine whether the observed patterns persist, intensify or shift as workers remain in their jobs. That is especially important in healthcare, where staffing pressures can keep workers on nights for long stretches and where schedule redesign is often constrained by service demands.
The cohort was built to support broader research use, and data will be made available for future studies. That openness increases the chance that investigators can compare EPHOR-NIGHT with other cohorts, test replicated mechanisms and refine prevention strategies around light exposure, shift rotation, sleep hygiene and work organization. The study does not yet deliver a disease endpoint verdict, but it creates the infrastructure needed to answer whether chronic exposure to night work leaves a measurable imprint on the biology of ageing and chronic disease.
The most actionable limitation is also the most obvious one: these are early cohort data, and the clinical endpoints of long-term disease progression are still ahead.