Roughly one in five workers worldwide performs shift work, nights, rotating schedules, early mornings that blur the line between one day and the next. The toll it takes on health has been documented for decades, but a growing body of evidence is now putting numbers to the cost: a 36 percent higher risk of dementia, measurable increases in systemic inflammation after just three consecutive nights of restricted sleep, and a cancer classification from the World Health Organization’s International Agency for Research on Cancer that puts night shift work in the same risk category as red meat.
The picture is sobering but not hopeless. Researchers are also beginning to identify what shift workers can do to protect themselves, and the answer may lie in a pattern of sleep that humans practiced for most of our history.
What the data shows
The most striking recent finding comes from a Swedish study at the Karolinska Institute that tracked more than 13,000 shift workers for up to 41 years. Published in the European Journal of Epidemiology, it found that those who had ever worked shifts had a 36 percent higher risk of developing dementia than those who had not. The risk increased with longer exposure, suggesting a dose-response relationship: more years of shift work, higher risk.
The mechanism may involve the brain’s glymphatic system, the waste-clearing network that flushes out metabolic byproducts, including amyloid and tau proteins associated with Alzheimer’s disease, during sleep. A Cambridge University study of roughly 40,000 adults from the UK Biobank found that people with impaired glymphatic drainage were significantly more likely to develop dementia years later, sometimes up to a decade before clinical diagnosis. Hugh Markus, the study’s lead researcher at Cambridge, said the findings suggest that even a single night of poor sleep can measurably raise amyloid levels in the brain.
The cardiovascular effects are equally concerning. A meta-analysis of 35 studies involving 887 participants, published in the Journal of Sleep Research, found that restricting sleep to roughly 4.5 hours for three or more consecutive nights significantly increased two key markers of systemic inflammation: interleukin-6 (IL-6) and C-reactive protein (CRP). Both are linked to cardiovascular disease, and the effect was large enough to be clinically meaningful.
Cancer risk is another well-documented concern. The IARC classified night shift work as “probably carcinogenic to humans” (Group 2A) in 2020, citing links to breast, prostate, colon, and colorectal cancers. The proposed mechanisms include suppression of melatonin, a hormone with tumor-suppressing properties, and disruption of circadian rhythms that regulate cell division and DNA repair.
The countermeasure no one talks about
Despite the risks, many shift workers have little choice about their schedules. Nurses, factory workers, emergency responders, and transport crews cannot simply opt out of night shifts. So researchers are looking for strategies that work within the constraints.
One promising line of inquiry involves biphasic sleep, splitting sleep into two distinct blocks rather than trying to get it all in one go. The idea is not new. Before the Industrial Revolution and the widespread adoption of gas lighting, humans commonly slept in two phases: a “first sleep” of roughly four hours, followed by an hour or two of wakefulness, then a “second sleep.” The historian Roger Ekirch at Virginia Tech documented this pattern extensively in pre-industrial European and American records. In a 1992 experiment, Thomas Wehr at the National Institute of Mental Health showed that volunteers placed in 14-hour periods of darkness naturally reverted to biphasic sleep within weeks.
Line Victoria Moen, a researcher at Norway’s National Institute of Occupational Health, is applying this insight to shift workers. Studying Arctic shift workers using Oura Ring sleep trackers, she has observed that many already sleep biphasically by instinct, a block from 9 a.m. to 1 p.m., followed by another block in the afternoon. Her ongoing scoping review of roughly 11,000 papers aims to produce the first evidence-based clinical guidelines for biphasic sleep in shift work.
Sleep as a pillar of health
Russell Foster, a circadian neuroscientist at the University of Oxford, sums up the emerging consensus: “Sleep is a pillar of our health, in the same way we think about diet and exercise. We have to take control of it.”
The practical advice for shift workers is still evolving, but researchers agree on the fundamentals. Planned napping during shifts, particularly naps of 20 to 50 minutes, improves alertness and reduces the risk of drowsy driving on the commute home. Managing cardiovascular risk factors, blood pressure, smoking, cholesterol, is especially important, since at least 25 percent of dementia risk comes from modifiable vascular factors. And recognizing that the body’s natural signals are not a weakness to be overcome but a guide to be worked with may be the most important mindset shift of all.
Sources
- Bokenberger, K., et al. “Shift work and risk of incident dementia: a study of two population-based cohorts.” European Journal of Epidemiology (2018). DOI: 10.1007/s10654-018-0430-8
- Hong, H., et al. “MRI markers of cerebrospinal fluid dynamics predict dementia.” Alzheimer’s & Dementia (2025). DOI: 10.1002/alz.70699
- Ballesio, A., et al. “Effects of experimental sleep deprivation on peripheral inflammation.” Journal of Sleep Research (2026). DOI: 10.1111/jsr.70099
- Moen, L.V., et al. “Biphasic sleep in shift workers and its related outcomes: a scoping review protocol.” BMJ Open (2026). DOI: 10.1136/bmjopen-2025-115567
- IARC Monographs Volume 124: “Night Shift Work” (2020)
- BBC News: “The hidden cost of the night shift and how to sleep it off“