London, Nov 17 (IANS): In a major study, researchers have revealed that shift workers, especially those working permanently in the night rotation, may be at heightened risk of moderate to severe asthma.

According to the study, published in the journal Thorax, around one in five employees in the developed world works permanent or rotating night shifts.

Shift work causes a person's internal body clock (circadian rhythm) to be out of step with the external light and dark cycle.

"This misalignment is associated with a heightened risk of various metabolic disorders, cardiovascular disease, and cancer," said study authors from University of Manchester in the UK.

Symptoms of asthma, such as wheeze and airway whistling, vary considerably, according to the time of day or night, and the researchers wanted to find out if shift work might also be associated with an increased risk of asthma and/or its severity.

They were also keen to explore how influential chronotype--individual body clock preference for morning or evening activity--and genetic predisposition to asthma might be.

They drew on medical, lifestyle, and employment information supplied between 2007 and 2010 by 286,825 participants in the UK Biobank.

All these participants were aged between 37 and 72, and either in paid employment or self-employed.

Most (83 per cent) worked regular office hours, while 17 per cent worked shifts, around half of which (51 per cent) included night shifts.

Shift patterns comprised: never or occasional night shifts; irregular or rotating night shifts; and permanent night shifts.

Compared with those working office hours, shift workers were more likely to be men, smokers, and living in urban areas and in more deprived neighbourhoods. They also drank less alcohol, slept fewer hours, and worked longer hours.

Night shift workers are considered to be 'owls' and generally have poorer health.

Some 14,238 (around five per cent) of all the study participants had asthma and in 4,783 (nearly two per cent) symptoms were moderate to severe (based on their medications).

The researchers compared the effect of working office hours with shift work on asthma diagnosis, lung function, and symptoms of asthma.

There was a 36 per cent increase in the odds of having moderate to severe asthma in permanent night shift workers compared to those working normal office hours.

Similarly, the odds of wheeze or airway whistling were 11-18 per cent higher among those working any of the three shift patterns, while the odds of poorer lung function were around 20 per cent higher in shift workers who never or rarely worked nights and those working permanent night shifts.