Shift work may not only make people grumpy, but it could put them at a higher risk of heart disease, Italian researchers said on Monday.

Alarm clocks and artificial lighting may fool the mind into thinking it is daytime and all right to work, but organs such as the heart march to a different tune, and do not respond well to being made to work in the middle of the night, a team at the University of Milan found.

Nerve activity that accelerates the heart is lower in people working an overnight shift as compared to morning or afternoon shifts, the researchers reported in the journal Circulation, published by the American Heart Association.

"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," Dr. Raffaello Furlan, who led the study, said in a statement.

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

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

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

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," Furlan said. "In this study we looked at two types of nerve systems that regulate the heart."

These are the vagal autonomic system, which slows down heart rate and lower blood pressure and body temperature at night, and 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.

They 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 -- a pattern that disregards changes in daily sleep 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.

"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," Furlan said.

"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."

They said many other studies have found that shift workers such as truck drivers are sleepier and make more mistakes at night.

"If I have to act or to perform a job task without such a high level of cortisol or body temperature, what we call the biophysical environment is less favourable and I am much more prone to make errors and to have accidents," Furlan said.

(From ChinaDaily)