A naturally-produced hormone that helps run the body's internal clock could be an effective blood pressure drug, say scientists from the Netherlands and US.
Melatonin is produced during sleep
The researchers found that they could reduce night blood pressure by giving a dose of melatonin just before bed.
The drug, often touted as a jetlag "cure", is linked to the natural cycle leading to drops in heart rate and blood pressure during the night.
However, experts say that it is too early for patients to start taking it.
Millions in the UK suffer from high blood pressure, many taking a variety of drugs to ease the problem - which carry some degree of side-effects.
Over a long period, high blood pressure has the potential to cause extra damage to blood vessels and organs, increasing the chance of life-threatening problems.
Scientists had already noticed that, during the night, the human heart rate lessens, as does blood pressure.
Some believe that this is not just happening because the person is asleep, but falls as part of a natural cycle, or circadian rhythm influenced by melatonin levels.
The researchers in this case, from the Institute for Brain Research in Amsterdam, tested 16 men with untreated high blood pressure.
They gave some a three week course of melatonin and others a placebo pill with no active ingredients.
They found that those taking the melatonin at night had significant reduction in blood pressure during that period.
Dr Frank Scheer, from Harvard Medical School, who coordinated the study, said: "This finding might open the door for a new approach for treating hypertension."
However, Dr Dan Jones, from the American Heart Association, said it was too early to recommend the use of melatonin in large numbers of patients.
He said: "Larger studies certainly would be needed prior to recommending this approach to patients with high blood pressure."
In the UK, Dr Nelson Chong, Coulson Lecturer in molecular cardiology at the University of Leicester, said that provided tests suggested that the chemical was non-toxic, it held promise as a potential treatment for hypertension.
He said: "Logic would suggest that it's certainly worth treating high blood pressure during the night. Without the blood pressure dip it will be like driving your car in low gear at high speed.
"Melatonin could be a very good candidate for future therapeutics, and it is one which has been pretty much overlooked previously, as most people have associated it only with sleep disorders.
"However, people with high blood pressure should not be going out and getting melatonin to take for this reason yet."
The study was published in the journal Hypertension.