I don't think you should worry too much about taking melatonin as long as you take it at the right time, an hour or two before you sleep if you take the pill version, or when you turn out the lights if you use a spray or sublingual. Very low doses are advised.
I have been talking melatonin on and off for about five years, and sometimes at a stretch, I also use an oestrogen patch and have not noticed any untoward effect.
Here is a scientific paper on melatonin
http://examine.com/supplements/Melatonin/ and here health benefits
http://articles.mercola.com/sites/articles/archive/2013/03/19/melatonin-benefits.aspx Also possible drug interactions
http://www.umm.edu/altmed/articles/melatonin-000970.htm A book I recommend is "The Melatonin Miracle by Walter Pierpaoli, M.D. and William Regelson, M.D" there is some good info on the connection between melatonin and menopause, there was a trial in Italy where melatonin was added to HRT.
It also lowers cortisol levels which in turn lowers stress so is good for the heart.