Why some people shake off a COVID-19 infection in days while others develop symptoms that linger for months has puzzled researchers since the earliest days of the pandemic. A new population-based cohort study published in the Scandinavian Journal of Work, Environment & Health points to an overlooked variable: how well, and when, people sleep.
Researchers at the Barcelona Institute for Global Health (ISGlobal) followed 2,941 adults in the COVICAT cohort in Catalonia, Spain, between 2021 and 2023. Among the 1,899 participants who were newly infected with SARS-CoV-2 during the study, 284 went on to report long-COVID symptoms over two years of follow-up. When the researchers looked at who those 284 people were, two sleep-related factors stood out.
Night Shifts Nearly Doubled the Risk
Compared with day workers, people working night shifts had an 88% higher risk of developing long COVID, even after the researchers adjusted for vaccination status, chronic disease, and other established risk factors. Separately, chronic insomnia in the general population was associated with a 42% increased risk of long COVID.
The two factors compounded each other. Night shift workers who also had chronic insomnia were 2.7 times more likely to develop persistent post-COVID symptoms than day workers with no sleep problems — a gap far larger than either factor alone would predict.
A Circadian Explanation for a Stubborn Mystery
"Our findings suggest that circadian disruption plays a role" in long COVID risk, said ISGlobal researcher Manolis Kogevinas, one of the study's authors. The working theory is mechanistic rather than incidental: disrupting the body's internal clock is known to weaken immune signaling and blunt the immune system's ability to fully clear a viral infection, potentially leaving low-grade inflammation or viral remnants that drive prolonged symptoms.
That would help explain a pattern that has frustrated long-COVID researchers for years — that initial infection severity alone does not reliably predict who develops lasting symptoms. Two people with similar initial illness can have very different outcomes, and this study suggests their sleep and work schedules in the months around infection may be part of why.
The researchers note that night shift work and chronic insomnia are both already independently linked to worse cardiovascular and metabolic health, so the long-COVID finding adds to a growing case that circadian disruption has systemic consequences well beyond daytime fatigue.
What This Means for Patients
This is an observational cohort study, so it cannot prove that fixing sleep problems would prevent long COVID — only that the two are statistically linked, with a plausible immunological explanation. Still, the researchers frame the practical takeaway as low-cost and low-risk regardless: shift workers and people with chronic insomnia already have reason to prioritize sleep for cardiovascular and metabolic health, and this study adds infection recovery to that list.
People who work night shifts and have ongoing trouble sleeping should not assume increased long-COVID risk is unavoidable. Standard tools for managing shift-work sleep disruption — consistent light exposure timing, protected sleep windows, and treating chronic insomnia with cognitive behavioral therapy rather than letting it go unaddressed — are the same interventions already recommended for other health reasons. Kogevinas noted that promoting healthy sleep habits could represent "simple, low-cost strategies to reduce long-term viral consequences," making this one more reason for shift workers and clinicians to take sleep complaints seriously rather than treating them as an unavoidable cost of the job.
