Can melatonin cause irregular heartbeat?

NOT usually. Melatonin is a hormone found naturally in the body. It's main use is for insomnia. Melatonin seems to be able to shorten the amount of time it takes to fall asleep, but only by about 12 minutes! (according to one research study) the usual dose 1-3mg. Melatonin can raise blood pressure in people who are taking certain medications to control blood pressure-so not recommended in hypertension (hi bp).