The one proven effect is you hear more scary stories. Whether there’s anything to them remains uncertain. So far, I’m not seeing a good reason to give up your night job.
Lately, most of the attention has focused on a possible link between night work and breast cancer. A 2001 Danish study of 7,000 women showed a 50 percent higher risk of breast cancer if they’d worked nights at least six months. A 2006 study of 45,000 Norwegian nurses working the night shift found the risk of breast cancer for some of them was more than double that of day-shift nurses.
Why might night work lead to more breast cancer? One theory points to the hormone melatonin, which is secreted by your pineal gland and helps regulate your body clock. Melatonin is produced most abundantly at night, but this can be substantially reduced by exposure to light, including artificial light while you’re on the job. Some researchers say suppressed melatonin levels can lead to an increase in sex hormones, which in turn can increase cancer risk. In support, they cite a couple of studies indicating profoundly blind women have only half as much chance of developing breast cancer as sighted women.
Scary? You bet. But let’s dive a little deeper into the numbers. The Danish investigators compared 7,000 women who had breast cancer with 7,000 women who didn’t, factored out things like age and socioeconomic status, then compared job schedules. The night workers seemingly got more than their share of cancer.
Trouble is, we’re not sure the “night workers” actually worked at night. The test subjects weren’t interviewed—their personal information was pulled from various databases and their schedules were inferred based on the likelihood of women’s doing night work in their various occupations, as determined by a prior survey.
The process was imprecise, the detected risk difference modest, the possibility of confounding influences difficult to rule out. For example, flight attendants, who were assumed to do a lot of night work, are exposed to more ionizing radiation, which can cause cancer. Were the results a fluke? Hard to say, but you’d want to see if similar studies came to the same conclusion.
Fine, you say. Bring on the 45,000 Norwegian nurses.
Well, on close examination, the Danish and Norwegian studies don’t show the same thing—they show opposite things. Remember, the Danish study claims increased cancer risk after just six months of working nights. The Norwegian study found nurses who’d worked nights for 14 or fewer years had a slightly lower cancer risk than day-shift nurses. The risk was just a bit greater for those who’d worked nights for between 15 and 29 years; the only nurses with double the cancer risk were those who’d worked nights for 30 years or more.
What’s more, just 24 of those nurses actually got cancer, and, as with the Danish study, we’re not entirely certain they worked nights.
You encounter the needle-in-a-haystack problem a lot in this kind of research. For example, a 2006 study of 14,000 Japanese men purports to show rotating-shift workers had triple the risk of prostate cancer. (Those working strictly at night had no significant risk increase.) Fourteen thousand sounds impressive, and here the researchers asked the workers themselves about when they’d been working. But how many cases of cancer is our frightening conclusion based on? Just 31.
I’m not seeing much reason to revise the conclusion of a 2003 review in the journal Occupational Medicine: “Today there is no conclusive evidence that night work per se increases the risk of cancer.” I’ll boldly say the risk of rotating-shift work is unproven too.
Of course, the big C isn’t the only alleged danger:
• One study found a third of night-shift workers suffered from insomnia and such, compared to less than a fifth of day workers.
• A German study found night-shift workers were 60 percent more likely to have had an ulcer and 70 percent more likely to have had gastrointestinal complaints.
• A study from Sweden correlated night-shift work with a poorer ratio of good to bad cholesterol. Other studies have noted a possible link between night work and weight gain.
But so far, dire reports like these haven’t added up to anything either. As that 2003 review puts it, “Today, no evidence exists showing that shift work affects longevity.”
Still, you never know. Common sense suggests night work is stressful; studies show the risk of on-the-job accidents is 30 percent higher at night. Science may someday succeed in penetrating the smoke and finding a fire. But it hasn’t yet.
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