Nightshifts linked to heart disease

The research was carried out on steel workers

People who work nightshifts as part of their job may be at greater risk of heart disease, say researchers.

A team from the University of Milan has found that the heart does not respond well to being made to work hard in the middle of the night.

This is because the body is programmed to slow down during the night - and changes to sleep patterns appear to have no effect.

The researchers found that nerve activity that accelerates the heart is lower in people working an overnight shift than when they work in the morning or afternoon.

It may be like if you start on a very cold day and you switch on your car and start out at maximum speed without letting the engine warm up

Dr Raffaello Furlan, University of Milan

Lead researcher Dr Raffaello Furlan said: "This resistance of the body's internal clock to change with varied work schedules indicates that people don't adapt as easily as we think to shift work, and could explain why shift workers are at higher risk.

"It seems to me that shift workers, because they have to change their shift every week, they cannot adapt completely to a different shift.

"This may explain some of the disorders they get."

Dr Furlan's team measured nerve activity that controls the heart in 22 male steel workers rotated through three different shifts - a night shift running from 10pm to 6am, an afternoon shift starting at 2pm and a morning shift that started at 6am.

Heart readings

They took electrocardiogram (ECG) recordings, which measure heart activity, after each worker had two days to adapt to the new shift.

The ECG measurements were taken constantly over a 24-hour period.

The heart rate is continuously changing on a beat-by-beat basis.

The study looked at two types of nerve systems that regulate the heart. These were:

• the vagal autonomic system, which slows down heart rate and lower blood pressure and body temperature at night
• the sympathetic nervous system, the part of the autonomic nervous system that prepares the body for the day's stresses by releasing hormones such as cortisol

The researchers also examined other studies that have used urine and blood samples to show changes in chemicals that affect the heart and other organs.

They found that nerve and chemical messages that control the heart's activity seem to follow a regular 24-hour pattern.

This pattern does not seem to be affected by changes to working patterns.

For example, levels of cortisol, a hormone that stimulates the heart rate, digestive system, breathing and other functions during the day, did not adjust to help night shift workers stay alert.

This could mean the heart is unprepared for the stress it will inevitably encounter during a work shift.

Dr Furlan said: "We don't know the mechanism. We just know that habitual shift work in a way may stress the heart.

"The numbers of cardiovascular diseases are increased among those people.

"It may be like if you start on a very cold day and you switch on your car and start out at maximum speed without letting the engine warm up."

More accidents

Other studies have found that shift workers such as truck drivers are sleepier and make more mistakes at night.

The disrupted sleep patterns of shift work could be a form of stress for our bodies

British Heart Foundation

A spokesman for the British Heart Foundation said it was possible that stress was linked to an increased risk of heart disease.

"The disrupted sleep patterns of shift work could be a form of stress for our bodies.

"As well as the physiological changes discussed in this research, a symptom of sleep stress could also be unhealthier lifestyles.

"Stress may cause people to rely more heavily on cigarettes, high fat foods and alcohol while finding less time for physical activity."

The research is published in Circulation, a journal of the American Heart Association.