Berlin: Living in the city can have a major effect on the internal clocks of both humans and animals which could lead to increased incidence of health problems and reduced lifespan, researchers have claimed.

Biologists from the University of Glasgow in UK and the Max Planck Institute for Ornithology in Germany have discovered that the biological rhythms of city-dwelling organisms are changing in response to city living.

The researchers measured the circadian rhythms, the 24-hour cycle of biological activity, of groups of urban and rural blackbirds in southern Germany and found that the city-dwellers had faster and less robust internal clocks than rural blackbirds. In the wild, city birds woke up earlier and rested less than forest birds.

The researchers captured adult male European blackbirds from the city of Munich and a nearby rural forest. Each bird was equipped with a lightweight radio-transmitter which monitored their daily levels of activity in the wild for 10 days before they were recaptured.

They were then kept in light-proof, sound-insulated chambers and their circadian rhythms were measured under constant conditions, without any environmental information that could serve as a 'clock'.

In this way, each bird's, internal rhythm could be tested. Once the tests were complete the birds were returned to the wild.

"The daily cycles of activity and rest are based on biological rhythms which have evolved as an adaptation to the rising and setting of the Sun," said Barbara Helm, of the University of Glasgow's Institute of Biodiversity Animal Health and Comparative Medicine.

"Our tests were designed to benchmark the internal rhythms of the birds under controlled conditions and to determine a link to the birds' chronotype in the wild. Chronotype is a measure of an individual's consistent timing relative to environmental factors, i.e., its relative 'morningness' or 'eveningness',” Helm said.

"We found that the rhythms of urban birds in the wild differ significantly from their forest counterparts. On average, they began their daily activities around 30 minutes before dawn, while forest birds began their day as the Sun rose,” he added.

"The city birds ended their days around nine minutes later, meaning they were active for about 40 minutes longer each day. In constant laboratory conditions, urban birds' circadian rhythms were clearly altered, running faster by 50 minutes than forest birds and being clearly less robust," Helm said.

The researchers have raised the possibility that the differences in the biological rhythms could be the result of micro-evolutionary changes in response to the stimuli of urban life such as artificial light and increased levels of noise.