New research provides further evidence of the harms of shift work, after finding it may increase stroke severity by interfering with the body's internal clock, with men faring worst.
Study co-author David Earnest, Ph.D., of the Department of Neuroscience and Experimental Therapeutics at Texas A&M Health Science Center College of Medicine, and colleagues publish their findings in the journal Endocrinology.
According to the Centers for Disease Control and Prevention (CDC), around 15 million Americans engage in shift work - defined as regularly working outside of the typical "9-to-5" pattern.
It is well known that such working patterns can interfere with the body's circadian rhythm, which is an approximate 24-hour cycle controlled by an internal biological clock.
The biological clock responds to external cues, such as light or dark, which tell the body when to sleep, wake, eat, among numerous other physiological processes.
"A person on a shift work schedule, especially on rotating shifts, challenges or confuses their internal body clocks by having irregular sleep-wake patterns or meal times," explains Earnest.
Many studies have highlighted the possible health implications of shift work. A recent study published in JAMA, for example, found a higher risk of heart disease among women who worked rotating night shifts for 5 years or more.
Earnest and colleagues build on such findings with their new study, suggesting shift work may also increase stroke severity.
Circadian rhythm disruption led to worse stroke outcomes in rats
The researchers came to their conclusion after analyzing the stroke outcomes of male and female rats that were subject to circadian rhythm disruption.
For 2 weeks, the rodents were exposed to a "fixed" light-dark condition, before being allocated to one of two groups.
For 7 weeks, one group had continued exposure to the fixed light-dark condition (the control group), while the other group experienced a significant shift in light-dark exposure - representing a shift work schedule.
The researchers then induced ischemic stroke in the rats and assessed their outcomes. Ischemic stroke is the most common type of stroke, where the artery that supplies blood to the brain becomes blocked.
The team found that the rats exposed to the shift work schedule had much more severe stroke outcomes than the control group. In detail, they were more likely to have brain damage, loss of sensation, and poorer limb movement as a result of ischemic stroke.
"This research has clear implications for shift workers with odd schedules, but probably extends to many of us who keep schedules that differ greatly from day to day, especially from weekdays to weekends.
These irregular schedules can produce what is known as 'social jet lag,' which similarly unwinds our body clocks so they no longer keep accurate time, and thus can lead to the same effects on human health as shift work."
Male shift workers have poorer stroke outcomes
Interestingly, the researchers found that stroke severity varied between male and female rats as a result of shift work, with males experiencing worse outcomes than females.
- Ischemic stroke accounts for around 87 percent of all strokes
- Stroke kills almost 130,000 Americans each year
- Stroke costs the U.S. around $34 billion annually.
The team speculates that this gender difference may be down to reproductive hormones.
"Young women are less likely to suffer strokes, as compared with men of a similar age, and when they do, the stroke outcomes are likely to be less severe," explains study co-author Farida Sohrabji, Ph.D., also of the Department of Neuroscience and Experimental Therapeutics at Texas A&M.
"In females, estrogen is thought to be responsible for this greater degree of neuroprotection."
The team notes, however, that previous studies have shown that older women approaching menopause tend to have more severe stroke symptoms than men.
Overall, the authors say their findings indicate that shift workers and other individuals with irregular sleep schedules should be monitored more closely for cardiovascular disease risk factors, such as high blood pressure and obesity.
Additionally, they recommend that shift workers adhere to lifestyle factors known to lower the risk of cardiovascular disease, such as a healthy diet, regular exercise, and avoiding tobacco use.
The team now plans to investigate whether the link between shift work and increased stroke severity may be down to inflammation in the brain.
"With this information, we may be able to identify therapeutic interventions that limit damage after a stroke in patients with a history of shift work," says Earnest.