WORKING all night to answer American phone calls does not sound healthy. “ You’re seated much of the time, and then you binge on junk food,” says Jose Mari Mercado, head of the main outsourcing association in the Philippines.  Call-centre workers try to catch up on lost sleep during the day, but often fail, and then flop at the end of the week. 

New research suggests that night work is very unhealthy indeed.  One study found that the longer nurses in South Korea had worked the night shift, the more likely they were to be obese.  Another study, of retired car workers in China, found that shift work was associated with high blood pressure and diabetes.  A French study in 2014 found that ten years of shift work was associated with cognitive decline equivalent to an extra six-and-a-half years of ageing. 

People who work at night suffer in two ways, says Derk-Jan Dijk of the University of Surrey.  First, a new schedule throws the body’s “circadian clock”—the inbuilt mechanism that regulates waking and sleeping—out of alignment.  Night workers eat when their bodies are not ready for food and try to sleep when they are not tired.  That leads to the second problem: night-shift workers simply do not sleep enough. 

It is hard to know whether sleep disruption or exhaustion causes ill-health—or both together.  A link between night work and type 2 diabetes, for example, might be because eating at the wrong times leads to more free fatty acids or because exhausted people eat more, or even because it can be hard to get wholesome food in the middle of the night. 

In theory, night workers could avoid health problems by completely switching to a night-time schedule.  But weekends, social obligations and sunlight make that impossible for most.  Mr Dijk says the only people who seem to manage it are shift workers on offshore oil rigs, who labour in windowless rooms and do not take weekends off.  But they suffer from jet lag when they return home.