Melatonin is fascinating stuff.
In addition to its use as a sleep aid, melatonin exerts possible effects on cardiovascular parameters, including anti-oxidative action on LDL, reduction in sympathetic (adrenaline-driven) tone, and reduction in blood pressure.
Several studies document the blood pressure-reducing effect of melatonin:
Daily nighttime melatonin reduces blood pressure in male patients with essential hypertension.
Melatonin reduces night blood pressure in patients with nocturnal hypertension.
Prolonged melatonin administration decreases nocturnal blood pressure in women.
Blood pressure-lowering effect of melatonin in type 1 diabetes.
But blood pressure may be increased when melatonin is added to nifedipine, a calcium channel blocker:
Cardiovascular effects of melatonin in hypertensive patients well controlled by nifedipine: a 24-hour study.
Effects on BP tend to be modest, on the order of 5-8 mmHg reduction in systolic, half that in diastolic.
But don't pooh-pooh such small reductions, however, as small reductions exert mani-fold larger reductions in cardiovascular events like heart attack and stroke. NIH-sponsored NHANES data (see JNC VII), for example, document a doubling of risk for each increment of BP of 20/10. The Camelot Study demonstrated a reduction in cardiovascular events from 23% in placebo subjects to 16.7% in subjects taking amlodipine (Norvasc) with a 5 mm reduction in systolic pressure, 2 mmHg drop in diastolic pressure. Small changes, big benefits.
Many people take melatonin at bedtime and are disappointed with the effects. However, a much better way is to take melatonin several hours before bedtime, e.g., take at 7 pm to fall asleep at 10 pm. Don't think of melatonin as a sleeping pill; think of it as a sleep hormone, something that simply prepares your body for sleep by slowing heart rate, reducing body temperature, and reducing blood pressure. (You may need to modify the interval between taking melatonin and sleep, since individual responsiveness varies quite a bit.)
I also favor the sustained-release preparations, e.g., 5 mg sustained-release. Immediate-release, while it exerts a more rapid onset of sleep, allows you to wake up prematurely, The sustained-release preparations last longer and allow longer sleep.
The dose varies with age, with 1 mg effective in people younger than 40 years, higher doses of 3, 5, even 10 or 12 mg in older people. Sustained-release preparations also should be taken in slightly higher doses.
The only side-effect I've seen with melatonin is vivid, colorful dreams. Perhaps that's a plus!