An experiment in which fruit flies were given regular doses of drugs could help scientists understand how humans get hooked.

Researchers were able to produce a genetically-modified fly that which proved unreactive to large quantities of cocaine.

Strangely, the genes which seem to affect this are those which control the fly's internal body clock.

Human fly

The similarity between the genetic make-up of the fruit fly and that of human being is well-known, and scientists are hoping that this breakthrough could eventually lead to understanding of addiction to narcotics.

Dr Jay Hirsch, of the University of Virginia, said: "These important findings illustrate that the clock genes perform other important roles in regulating the physiology of fruit flies, and likely humans."

He added: "This opens up the field of drug studies to thinking about how a totally unexpected set of genes functions in response to drugs."

The research team took fruit flies and removed several genes that regulate their internal biological clock, which, in humans, decides when we wake up and go to sleep.

They then found that increasing doses of cocaine had no effect on the flies.

Dr Alan Leshner, Director of the National Institute on Drug Abuse, said: "This exciting new research has given us a clue to the specific genetic mechanisms that influence vulnerability to addiction.

"These mechanisms could become the basis for predicting who is at most risk for addiction and thus become a major aid in preventing this national health problem."

Research into the effects of the human body clock is continuing in many different areas, which may help scientists understand why heart attacks are more likely to strike in the morning and asthma attacks at night.