The hormone is currently available in the US and countries such as Singapore and Thailand as a 'dietary supplement'. In Europe, Australia and many other countries however it is a regulated as a medicine.
A previous study from Harvard Medical School reported that taking melatonin for three-weeks reduced the systolic BP of men with hypertension by six millimeters of mercury (mmHg), and diastolic BP by four mmHg.
The new study, published in the American Journal of Hypertension (Vol. 18, pp. 1614-1618), supports the hypothesis that melatonin reduces BP.
Although the study was limited to 18 middle-aged women, half of whom had hypertension, the results strongly indicate that prolonged intake of melatonin could improve the day-night BP rhythm.
The release of melatonin during the night has been related to the typical blood pressure drop during sleep. The current study sought to test if melatonin supplements could amplify the nighttime fall in BP.
The randomized, double blind, placebo-based study gave nine women either a placebo or a melatonin pill one hour before bedtime. The melatonin pill was formulated to quickly release 1mg of melatonin, and a further 2mg slowly. After three weeks of intervention the groups were crossed over.
"A 10 per cent decline of BP over daytime values is considered appropriate to reduce the cardiovascular risk," said lead-author Angelo Cagnacci. "During placebo use only 39 per cent of the subjects reached this value, but this rate surged to 84 per cent during melatonin administration."
The mechanism whereby melatonin reduces BP is unknown, but the researchers propose it is not merely due to the sleep-inducing effect of the hormone.
"Melatonin may exert cardiovascular effects that are independent of those on sleep," said Cagnacci.
The authors recommended that the interactions with other antihypertensive therapies, which were not excluded or evaluated in this study, should be investigated.
"Longer clinical trials are necessary for a full exploration of the potential effects of melatonin," concluded Cagnacci.
In 2003 UK scientists from the Cochrane Collaboration called for funding to back proper testing of melatonin in supplement form.
The market for natural sleep aids could be important. Over 12 million sleeping pill prescriptions are issued every year in the UK. In the US, over 70 million people are said to have sleeping problems.
The Committee on the Framework for Evaluating the Safety of Dietary Supplements published prototype monographs in 2003 for three supplements including melatonin, and is asking for comment and contributions to its information.