A hormone supplement taken by poor sleepers - or to avoid jetlag - could threaten asthmatics, say researchers.
Nocturnal asthmatics have more melatonin
Melatonin is a hormone produced in a gland in the brain, and helps maintain the natural cycle of sleep and wake.
Researchers from the National Jewish Medical and Research Center in Denver, US, found that the hormone caused a drop in lung function in asthmatics.
They say that asthmatics should considering limiting the quantity of melatonin supplements they take.
Melatonin is normally released by the body at night, starting in the evening and peaking in the middle of the night.
People who have trouble getting a good night's sleep take it, as do those who are badly affected by jet lag.
It has even been tested as a possible treatment for certain types of cancer.
However, it is not freely available in the UK, although many people still obtain it through mail-order or internet sites.
So far, safety studies have suggested it is not toxic, but the latest research, published in the Journal of Allergy and Clinical Immunology, studied the effect on the lungs and breathing.
It is already known that melatonin induces the release of chemicals that can increase inflammation in the cells lining the airways.
In tests on seven people with nocturnal asthma, 13 non-nocturnal asthmatics and 11 non-asthmatics, lung function tests revealed that patients with nocturnal asthma had the highest levels of melatonin and the greatest drop in lung function overnight.
The suggestion from scientists was that the drop in lung function was linked at least in part to the high levels of melatonin in their system.
Dr Rand Sutherland, who led the study, said: "For patients whose asthma worsens at night, we found that higher levels of naturally-occurring melatonin are associated with impaired lung function.
"Those findings, on top of previous ones linking melatonin to inflammation, suggest that all patients with asthma should avoid using melatonin supplements until more is known about their safety."