Shift work is the working of any hours outside of normal daytime hours. Unfortunately, since many people do it, shift work is bad for health.
The most common variations in shift work are the “swing shift”, work starting in mid-afternoon and ending late at night, and the night or “graveyard” shift, starting late at night and ending early in the morning.
I’ve worked plenty of both of these shifts in my life, and may have spent more hours working swing shifts than on the day shift. These hours may have contributed to my former ill health — in fact I think it likely.
A great deal of research has found associations between shift work and a number of diseases, notably heart disease and cancer.
A study that looked at workers in a paper mill found that increasing exposure to shift work styrongly increased the risk of heart disease.1
Those who had worked more than 11 years on shifts other than days had more than twice the risk of heart disease, and those who worked more than 16 years had almost triple the risk.
So you can see that the risk isn’t small.
Shift work is associated with cancer too. For instance, women who worked the night shift had a 60% increased risk of breast cancer.2
Shift work has been extensively studied, and all of the studies haven’t found such a huge increase in risk as those cited here. Others have found differences in smoking rates and dietary habits that could account for much of the difference. For example, it seems likely that shift workers eat more junk food than those on the day shift. Often, the only food to be found on a night shift comes from a vending machine.
But there are good reasons to think that shift work in itself is harmful.
A recent study written up in Scientific American found that bright light speeds up aging in mice.3 The mice experienced a decrease in bone density, more inflammation, and muscle weakness. Restoring the mice to health was a simple matter of turning off the bright lights at night.
Exposure to light at night disrupts circadian rhythms, which are so important for health. One of the main consequences of light exposure at night is that it greatly disrupts the production of melatonin, a hormone that promotes sleep and good immune function.4
The disruption of melatonin is particularly sensitive to blue light, i.e. the kind that comes from artificial lighting and from computer screens and phones.
Shift work and exposure to light at night are associated with cancer and heart disease.