Increasing numbers of travel-weary Brits are stocking up on remedy for jet lag which is still not widely available in the UK.
Long-haul flights disrupt the body
Melatonin is sold over-the-counter in the US to help with sleep disorders and jetlag. It is also widely available on the internet.
Many people swear by the benefits, but others have pointed to evidence that the hormone can produce side effects, including a drop in lung function in asthmatics.
And in the UK it is "off licence" - only available on prescription if the doctor who is prescribing is prepared to take personal liability for any side effects.
The Department of Health says that no UK company has yet applied for a marketing license.
But while in the US melatonin is sold in pharmacies as a dietary supplement, in the UK it is considered to be a medicine.
The reason for this conflicting opinion is that as well as being available in pill form, melatonin is also produced naturally by the pineal gland of the brain and is present in certain foods.
Its functions are vital to the regulation of the body's "circadian rhythms", whose powerful clock-like mechanisms synchronise the body with the 24 -hour period that is day and night.
When melatonin production is disturbed there are a number of side effects.
Production of the chemical at night stimulates the onset of sleep.
But during a light, long-haul flight, this production is inhibited, disrupting sleep, throwing the body completely out of sync and leading to jet lag.
The idea of using supplements, at least in the US, is that by taking exogenous melatonin at appropriate times, you can succeed in resetting your body's clock and beat jet lag.
However, it important to get the dosage exactly right and for it to be taken within correct time intervals.
One side effect is fatigue and so an incorrect dosage can actually exacerbate the original condition.
Dr Peter Abrahams, a London-based GP, who travels frequently, is a self-professed convert.
He stocks up with tablets for his own personal use when he is on the other side of the Atlantic.
He said: "I usually start taking it three days before coming back to England when my jet lag is much worse than when I fly out.
"The right amount of melatonin re-programmes your body, allowing you to have a normal night's sleep, without waking up."
However, Angela Falaschi, a nutritionist at London's reputed Hale Clinic, says the UK is right to adopt a far more cautious approach.
She prescribes changes in diet and exercise, which she says can make a real difference to jet lag and believes melatonin supplements should only be taken as a last resort.
"With hormones you have to be very careful, I would not recommend taking them without supervision as no one knows the side effects of taking melatonin.
"Some experts say, for example, that it might be dangerous for people with epilepsy or Parkinson's and that it stimulates white blood cell production and can cause leukaemia".
The only real certainty is that more research needs to be done.