For long-haul travellers it is said to take a day to recover for every time zone crossed, but now a drug based on the hormone melatonin has been shown to be an effective treatment for jet lag.

The drug, tasimelteon, made it easier for volunteers in a clinical trial to get to sleep, and helped them to stay asleep for longer, after their sleeping pattern was disrupted to mimic long distance travel.

The authors of the study, from Harvard Medical School in the US and Monash University in Australia, say the drug "could be a first-line therapy for people burdened with the effects of travel across time zones or working at night".

Melatonin is a natural hormone produced in the brain that helps to set the body clock, which controls the sleep-wake cycle.

The findings, based on separate studies involving 450 people in sleep laboratories at 21 sites in the US, are published in The Lancet. The authors say: "Tasimelteon has the potential for the treatment of patients with transient insomnia associated with circadian rhythm sleep disorders, including people affected by jet lag."

But a sleep expert said there was no way of telling if the new drug was more effective than the hormone, because it had not been tested against it. Irshaad Ebrahim, of the London Sleep Clinic, said drugs like tasimelteon, collectively known as melatonin agopnists, had "about the same effect" as melatonin.

Dr Ebrahim said: "You can't get melatonin in Britain because it has not undergone clinical trials. But you can walk into health food stores in the US, or South Africa, or Australia and buy as much as you like."

He added: "People don't take melatonin seriously because it is a health food supplement. So drug companies have said, 'let's find something that has a similar effect that we can register as a medicine'."

In the UK, the drug Circadin, which contains melatonin, was licensed this year for adults over 55 with sleep disorders.