Circadian · Diabetes
More years of rotating night shifts, more type 2 diabetes — in two large cohorts
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Short answer
Longer rotating night-shift work was associated with higher type 2 diabetes incidence in both Nurses' Health Study cohorts, with much of the association running through BMI.
What happened
Pan, Schernhammer, Sun and Hu tracked incident type 2 diabetes against self-reported years of rotating night-shift work (at least three nights a month) in the Nurses' Health Study I and II. There were 6,165 incident cases in NHS I and 3,961 in NHS II. Against never working nights, 20 or more years carried an age-adjusted hazard ratio of 1.64 in NHS I and 2.50 in NHS II. Adjusting for BMI substantially attenuated the association.
Why it matters
The duration-response gradient appearing separately in two cohorts is what distinguishes this from a single correlational finding. What it cannot distinguish is whether circadian disruption acts directly or largely through weight gain — BMI sits on the causal path, so adjusting for it removes part of the effect being studied.
Evidence
- Study type
- Two prospective cohort studies (Nurses' Health Study I and II)
- Sample
- 69,269 women in NHS I and 107,915 in NHS II; 6,165 and 3,961 incident diabetes cases
- Journal
- PLoS Medicine · peer reviewed
- Replication
- The duration-response pattern repeats across the two independent cohorts within this paper
- Limitations
- Shift history is self-reported and recalled. BMI both confounds and mediates. Female nurses may not represent all shift workers, and nobody can be randomised to years of night work.
What this connects to
Sources
The one study this explanation is built from, by the role each plays. Every source links to PaperFren’s explanation of it and to the original paper.
Primary study
- Night shifts and type 2 diabetes risk
Longer rotating night-shift work associated with higher type 2 diabetes risk in two large female nurse cohorts.
What it does not showLimitations
Self-reported work history and residual confounding limit causal certainty; nurses may not represent all workers.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Before
Circadian disruption was linked to metabolic dysfunction in laboratory studies, but cohort evidence on real shift schedules and hard diabetes endpoints was thin.
Now
Two independent prospective cohorts show a graded association with years worked. The design cannot establish causation, and the mediating role of weight gain is unresolved.