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Research · Daily life

Shift work

Pooled cohort and cross-sectional studies link shift work to more heart attacks, type 2 diabetes and obesity, with risk rising with years of exposure. About a quarter of shift workers met criteria for shift work disorder. IARC classes night shift work as probably carcinogenic, while prospective studies found no rise in breast cancer. Trials of remedies are small.

Checked 2026-11-13
Status
A daily life factor, not a product.
Also called
night shift, night shifts, night work, rotating shifts, rotating night shift work, graveyard shift, shiftwork, irregular work hours, shift work disorder, shift work sleep disorder, working nights
Studied for
heart health, diabetes, weight gain, sleep, alertness at work, energy, cancer risk

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Shown to help2

Reported risks5

What it was studied for, and what was found

Shown to help2

Limited

Redesigning shift schedules

Limited evidence: switching to faster or forward-rotating shifts, or letting workers choose their shifts, improved health or work-life balance in small, mostly uncontrolled studies.

Compared with the previous schedule, in experimental and quasi-experimental studies with or without a control group [1]

The numbers and sources
Effect
Across 26 studies, no type of schedule change was consistently harmful. Three types had beneficial effects on health, work-life balance or both: switching from slow to fast rotation, changing from backward to forward rotation (day, then evening, then night), and self-scheduling of shifts [1].
Population
Shift workers in 26 experimental and quasi-experimental studies from any country [1].
Sources
[1]
Caveats
The review gave a narrative synthesis with no pooled numbers, and the authors raised concerns about how far the evidence generalizes [1].
LimitedStudied in women

Heart disease after leaving night shift work

One cohort of women nurses linked more years since leaving rotating night shift work to lower coronary heart disease risk.

Compared with nurses with a history of shift work who had left it more recently [3]

The numbers and sources
Effect
Among women who had ever worked rotating night shifts, longer time since quitting shift work was linked to lower coronary heart disease risk in the younger cohort (P for trend under 0.001). In the older cohort, the link between years of shift work and heart disease was stronger in the first half of follow-up, which the authors read as waning risk after stopping [3].
Population
189,158 initially healthy women nurses in the Nurses' Health Studies, followed 24 years, with 10,822 coronary heart disease cases [3].
Sources
[3]
Caveats
Observational, in one occupation. Nurses who leave shift work may differ in health and income from those who stay. The abstract reports a trend, not a size [3].

Mixed results1

Moderate

Bright light, naps and other measures for sleepiness on shift

Mixed results: in a Cochrane review, bright light at night lowered sleepiness slightly in two studies, while naps, exercise and sleep education gave inconclusive results.

Compared with normal light [2]; control conditions the review abstract does not describe, for naps and other measures [2]

The numbers and sources
Effect
Bright light during the night shift lowered sleepiness by 0.83 points on the Stanford Sleepiness Scale (95% CI -1.3 to -0.36, very low-quality evidence). Another trial found no difference on another scale, and bright light during day shifts did not reduce sleepiness. A single nap opportunity changed reaction time by 11.87 ms, not significant. Exercise and sleep education showed promise without enough data to judge [2].
Population
556 shift workers who also work nights, in 17 randomized trials [2].
Sources
[2]
Caveats
Most comparisons gave low to very low-quality evidence and studies were too varied to pool [2].

Amounts studied

Rotating night shift work was defined as at least 3 night shifts a month on top of day and evening shifts [3]. Studies compared shift workers with day workers or the general population [4]. Dose was counted in years of shift work, with risk estimated per 5 extra years [5], and by schedule type, such as night, rotating, mixed and evening shifts [6]. One review estimated diabetes risk per 5 extra years in women [7].

Schedule studies tested fast against slow rotation, forward against backward rotation, and self-scheduling [1]. Trials for sleepiness used bright light during night or day shifts, one or two nap opportunities during the night shift, exercise and sleep education [2].

Risks

Reported risks4

ModerateVery common

Shift work disorder

About a quarter of shift workers met criteria for shift work disorder, insomnia or sleepiness tied to the work schedule, in one meta-analysis of 29 studies.

The numbers and sources
How common
Very common: more than 1 in 10 people.
Effect
The pooled prevalence of shift work disorder was 26.5% (95% CI 21.0 to 32.8) [11].
Population
Workers in 29 occupational studies [11].
Sources
[11]
Caveats
Heterogeneity was very high (I² 98.5%), partly explained by diagnostic criteria and sample size. Most studies had no day-worker comparison group [11].
StrongHow often is not known

Heart attack and coronary heart disease

Linked to about a quarter more heart attacks and coronary events in shift workers in pooled studies, with risk rising after the first 5 years of shift work.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Shift work was linked to myocardial infarction (RR 1.23, 95% CI 1.15 to 1.31), ischemic stroke (RR 1.05, 95% CI 1.01 to 1.09) and coronary events (RR 1.24, 95% CI 1.10 to 1.39). It was not linked to death from vascular or any cause. Every schedule except evening shifts carried a higher coronary risk [4]. A second review found 17% more cardiovascular events and 26% more coronary heart disease (RR 1.26, 95% CI 1.10 to 1.43). After the first 5 years, risk rose 7.1% for every further 5 years of shift work [5]. In nurses, more years of rotating night shifts were linked to more coronary heart disease, a small absolute increase [3].
Population
2,011,935 people in 34 studies [4]. 173,010 participants in 21 studies [5]. 189,158 women nurses [3].
Sources
[4], [5], [3]
Caveats
Observational. Shift workers differ from day workers in income, smoking and job type, though adjustment for smoking and socioeconomic status did not explain the heterogeneity [4]. Heterogeneity was high for coronary events (I² 85%) [4]. The two reviews disagree on cardiovascular death: one found no link [4] and the other a higher rate in a subgroup (RR 1.22) [5].
StrongHow often is not known

Type 2 diabetes

Linked to about 10% more type 2 diabetes in shift workers in three meta-analyses, with risk rising with years of shift work in women.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Ever working shifts was linked to diabetes (OR 1.09, 95% CI 1.05 to 1.12), more so in men (OR 1.37) and in rotating shifts. Every schedule except mixed and evening shifts carried a higher risk [6]. Cohort studies gave an RR of 1.14 (95% CI 1.10 to 1.19), and each 5 more years of shift work raised risk 7% in women (RR 1.07, 95% CI 1.04 to 1.09) [7]. A third review found an RR of 1.10 (95% CI 1.05 to 1.14) for type 2 diabetes, with higher risk in women and health care workers [8].
Population
226,652 participants with 14,595 cases in 12 studies [6]. 244,266 participants with 15,906 cases in 12 cohorts [7]. 12 cohort and 9 cross-sectional studies [8].
Sources
[6], [7], [8]
Caveats
Observational. The reviews disagree on whether the link is stronger in men [6] or women [8]. Diet, sleep loss and weight may lie on the path between shift work and diabetes.
StrongHow often is not known

Overweight and obesity

Linked to about 17% to 25% more overweight and obesity in shift workers in two meta-analyses, with the strongest link in permanent night workers and for abdominal obesity.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
Shift work was linked to overweight (RR 1.25, 95% CI 1.08 to 1.44) and obesity (RR 1.17, 95% CI 1.12 to 1.22) [9]. A second review found an OR of 1.23 (95% CI 1.17 to 1.29) for overweight or obesity, 1.10 in cohort studies and 1.35 for abdominal obesity. Permanent night workers had an OR of 1.43 against 1.14 for rotating shift workers [10].
Population
311,334 participants in 26 studies, 7 of them cohorts [9]. 28 studies [10].
Sources
[9], [10]
Caveats
Mostly cross-sectional studies, which cannot show which came first, and cut points for overweight varied widely [9]. Cohort studies gave the smallest link [10].

Mixed results1

ModerateHow often is not known

Cancer

Mixed results: IARC classes night shift work as probably carcinogenic, mainly from case-control and animal studies, while ten prospective cohorts found no rise in breast cancer.

The numbers and sources
How common
How often is not known: the sources give no rate.
Effect
IARC found limited evidence in humans, with positive associations between night shift work and cancers of the breast, prostate, colon and rectum in the largest case-control studies but inconsistent results in cohort studies. It found sufficient evidence in animals for altered light-dark schedules, and classed night shift work as probably carcinogenic to humans (Group 2A) [12]. In 10 prospective studies, any night shift work had a breast cancer RR of 0.99 (95% CI 0.95 to 1.03), and 20 or more years an RR of 1.01 (95% CI 0.93 to 1.10) [13].
Population
Studies of cancer in exposed people and lifetime studies in mice, reviewed in 2019 [12]. 1.4 million women in 10 prospective studies, 4,660 breast cancers in night shift workers [13].
Sources
[12], [13]
Caveats
IARC gave case-control studies more weight because they measured shift work in more detail [12]. Group 2A describes the strength of evidence that a hazard exists, not the size of any risk.

Claims tested and not found

  1. That long-term night shift work raises breast cancer risk. In 10 prospective studies, 20 or more years of night shift work had a breast cancer RR of 1.01, and 30 or more years 1.00 [13].
  2. That every kind of shift work carries the raised heart and diabetes risk. Evening shifts were not linked to more coronary events [4] or diabetes [6].

Related

Sleep durationMelatonin

Sources

  1. Bambra CL, Whitehead MM, Sowden AJ, Akers J, Petticrew MP. Shifting schedules: the health effects of reorganizing shift work. American Journal of Preventive Medicine. 2008. https://doi.org/10.1016/j.amepre.2007.12.023
  2. Slanger TE, Gross JV, Pinger A, et al. Person-directed, non-pharmacological interventions for sleepiness at work and sleep disturbances caused by shift work. Cochrane Database of Systematic Reviews. 2016. https://doi.org/10.1002/14651858.CD010641.pub2
  3. Vetter C, Devore EE, Wegrzyn LR, et al. Association Between Rotating Night Shift Work and Risk of Coronary Heart Disease Among Women. JAMA. 2016. https://doi.org/10.1001/jama.2016.4454
  4. Vyas MV, Garg AX, Iansavichus AV, et al. Shift work and vascular events: systematic review and meta-analysis. BMJ. 2012. https://doi.org/10.1136/bmj.e4800
  5. Torquati L, Mielke GI, Brown WJ, Kolbe-Alexander T. Shift work and the risk of cardiovascular disease. A systematic review and meta-analysis including dose-response relationship. Scandinavian Journal of Work, Environment and Health. 2018. https://doi.org/10.5271/sjweh.3700
  6. Gan Y, Yang C, Tong X, et al. Shift work and diabetes mellitus: a meta-analysis of observational studies. Occupational and Environmental Medicine. 2015. https://doi.org/10.1136/oemed-2014-102150
  7. Li W, Chen Z, Ruan W, et al. A meta-analysis of cohort studies including dose-response relationship between shift work and the risk of diabetes mellitus. European Journal of Epidemiology. 2019. https://doi.org/10.1007/s10654-019-00561-y
  8. Gao Y, Gan T, Jiang L, et al. Association between shift work and risk of type 2 diabetes mellitus: a systematic review and dose-response meta-analysis of observational studies. Chronobiology International. 2020. https://doi.org/10.1080/07420528.2019.1683570
  9. Liu Q, Shi J, Duan P, et al. Is shift work associated with a higher risk of overweight or obesity? A systematic review of observational studies with meta-analysis. International Journal of Epidemiology. 2018. https://doi.org/10.1093/ije/dyy079
  10. Sun M, Feng W, Wang F, et al. Meta-analysis on shift work and risks of specific obesity types. Obesity Reviews. 2018. https://doi.org/10.1111/obr.12621
  11. Pallesen S, Bjorvatn B, Waage S, Harris A, Sagoe D. Prevalence of Shift Work Disorder: A Systematic Review and Meta-Analysis. Frontiers in Psychology. 2021. https://doi.org/10.3389/fpsyg.2021.638252
  12. IARC Working Group on the Identification of Carcinogenic Hazards to Humans. Night Shift Work. IARC Monographs on the Identification of Carcinogenic Hazards to Humans, Volume 124. 2020. https://publications.iarc.who.int/593
  13. Travis RC, Balkwill A, Fensom GK, et al. Night Shift Work and Breast Cancer Incidence: Three Prospective Studies and Meta-analysis of Published Studies. Journal of the National Cancer Institute. 2016. https://doi.org/10.1093/jnci/djw169

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