City living may alter our daily body clock, making us active for longer and less rested, a study of birds has suggested.
Researchers compared the internal rhythms of blackbirds living in the countryside and in urban areas and found they differ significantly.
The city birds began their days earlier and ended them later, being active for around 40 minutes longer than the rural blackbirds.
Internal clocks of the city-dwelling birds were "less robust" and more prone to disturbance, the researchers said.
The differences in urban birds may have developed in response to artificial light and increased noise, they said.
The study team captured adult male blackbirds from Munich and from a nearby rural forest. They fitted each one with a lightweight radio-transmitter which monitored their daily activity in the wild for 10 days before the birds were recaptured.
They were then kept in light-proofed, sound-insulated chambers and their rhythms measured without any environmental information that could serve as a "clock". Once the tests were complete, the birds were returned to the wild.
The research, a collaboration between Glasgow University and the Max Planck Institute for Ornithology in Germany, is published in the latest edition of the journal Proceedings Of The Royal Society B.
Barbara Helm, from Glasgow University's Institute of Biodiversity, Animal Health and Comparative Medicine, said: "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.
"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. 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 (daily) rhythms were clearly altered, running faster by 50 minutes than forest birds and being clearly less robust. There seems to be a different beat to city life.
"Our work shows for the first time that when sharing human habitats, a wild animal species has a different internal clock.
"We'd be keen to find out the costs and benefits of modifying biological rhythms in blackbirds and other animals commonly found in our cities. This may help us to better understand the challenges of coping with urban life."
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