The longer you work night shifts, the higher your cardiovascular risk — switch if you can

Switching jobs or changing shifts may mean a pay cut and loss of night-shift bonuses. Job hunting takes time and determination. If you factor this into your calculations when choosing a job, the cost is effectively zero.

A pooled analysis of 23 studies — all observational, no subgroup adjustments — shows that night-shift workers face a roughly 13% higher risk of cardiovascular events (RR 1.13, 95% CI 1.10–1.16). Cardiovascular deaths rise by about 27% (RR 1.27, 1.18–1.36). Over time, each extra five night shifts adds roughly 7% to cardiovascular disease risk (RR 1.07, 1.04–1.09) and about 5% to cardiovascular mortality (RR 1.05, 1.03–1.06). Specific conditions show similar trends: coronary heart disease incidence rises by 22% (RR 1.22, 1.16–1.28) and mortality by the same amount; ischemic heart disease deaths increase by 39% (RR 1.39, 1.06–1.84); stroke mortality climbs by 49% (RR 1.49, 1.04–2.12). Stroke incidence, however, shows no significant rise (RR 1.06, 0.95–1.18).

Xi J, Ma W, Tao Y, 等 (2025). Association between night shift work and cardiovascular disease: a systematic review and dose-response meta-analysis. Frontiers in Public Health. https://doi.org/10.3389/fpubh.2025.1668848；Esposito G, Bravi F, Santucci C, 等 (2025). Night shift work and breast cancer risk in healthcare workers: a systematic review and meta-analysis. Occupational Medicine. https://doi.org/10.1093/occmed/kqaf040；Shen QM, Li ZY, Tan YT, 等 (2026). Night shift work and risk of total and site-specific cancer: results from a prospective cohort study among Chinese men. Scandinavian Journal of Work, Environment & Health. https://doi.org/10.5271/sjweh.4290；Czeisler CA, Johnson MP, Duffy JF, 等 (1990). Exposure to bright light and darkness to treat physiologic maladaptation to night work. New England Journal of Medicine, 322(18), 1253-1259. https://doi.org/10.1056/NEJM199005033221801；Eastman CI, Stewart KT, Mahoney MP, Liu L, Fogg LF (1994). Dark goggles and bright light improve circadian rhythm adaptation to night-shift work. Sleep, 17(6), 535-543：「Both bright light and goggles were significant factors for producing circadian rhythm phase shifts. The combination of bright light plus goggles was the most effective」「Goggles were necessary for producing phase delays」. https://doi.org/10.1093/sleep/17.6.535；Crowley SJ, Lee C, Tseng CY, Fogg LF, Eastman CI (2003). Combinations of bright light, scheduled dark, sunglasses, and melatonin to facilitate circadian entrainment to night shift work. Journal of Biological Rhythms, 18(6), 513-523：「With bright light during the night shift, almost all of the earlier participants achieved complete re-entrainment, and the phase delay shift was so large that darker sunglasses and melatonin could not increase its magnitude」. https://doi.org/10.1177/0748730403258422

These are all studies that observe participants without random assignment; people working night shifts are inherently different from others in terms of smoking, weight, and income, so the adjustments are not perfectly clean. The evidence regarding night shifts causing cancer is much weaker than the common saying. In an analysis summarizing 12 studies involving 12,132 cases of breast cancer, those who have worked night shifts had a risk only about 5% higher, and the confidence interval crossed 1 (RR 1.05, 0.96–1.14). For the category of having worked a full 20 years, the risk was about 25% higher (1.25, 1.01–1.55). However, after the author performed an impact analysis, this item was no longer significant. After adjusting for potential publication bias, it approached 1; the author's original words were that this association is "far from established." In another study tracking 8,202 cancer cases in Chinese men for 16.1 years, there was no association between night shifts and total cancer or most common cancers. Only pancreatic cancer was approximately 59% higher in the category of having cumulatively worked 11 to 20 years (HR 1.59, 1.09–2.31). Therefore, this item is calculated based on cardiovascular health, not cancer. This book does not advocate that everyone should leave night shift positions; it simply presents this calculation based on years of service so you can calculate it for yourself. For those already working shifts, change what can be changed first: for smoking cessation, see item 1 of this section (quit smoking, as early as possible); for blood pressure and blood lipids, see item 12 of this section (if you have hypertension or hyperlipidemia, take medication regularly as prescribed by a doctor); for how to catch up on sleep after a night shift, see item 38 of this section (catch up on sleep on the night following a late night). There are two other strategies worth trying: concentrating food intake during the day, and using bright light combined with strict light blocking during the day to shift the biological clock. The latter truly shifted the biological clock in experiments. Under ordinary indoor lighting, working six consecutive night shifts left the biological clock unmoved; switching to bright light of 7,000 to 12,000 lux combined with near-total darkness during the day caused the entire clock to flip in just four days. Researchers recommend doing three things together: night light must be bright enough, daytime sleep must involve strict light blocking, and sunglasses should be worn on the way home from work to block morning light. In a simulated night shift experiment with 50 people, bright light at night and sunglasses after work were each effective, and the combination of the two worked best. For those not wearing sunglasses, the biological clock only shifted toward an earlier phase and could not shift toward a delayed phase; the author believes sunlight on the way home from work was obstructing it. In another experiment with 67 people, those whose biological clocks were originally early almost all had their clocks completely flipped with bright light at night, and using darker sunglasses provided no further shift. For those with only indoor lighting, the darker the sunglasses, the more useful they were. The endpoint of both strategies only reaches metabolic indicators and sleep; no experiment has measured whether following these steps can reduce cardiovascular risk. Returning to a daytime lifestyle on days off will push the clock back again. Dosages, original numbers, and other sources, along with how the body recognizes time, are all in docs/生物钟和夜班.md.