Can passion flower and melatonin be used together?

Yes they both indeed can be used in conjunction with each-other in Naturalistic Treatment. Even taken together is fine. I would not exceed 10mg of melatonin unless your tolerance is high. Either by the nature of your body or by prior use with a end result of the body increasing how you tolerate it. Passion Flower is also very harmless unless your using a very strong ethanolic extract or more then 10g/daliy of 1:1 ratio of the herb in its nature form. If you are seeking restful sleep or the comfort of simple relaxation without anxiety some other helpful natural things to try are Bach Flower Extracts, GABA, valerian root, skullcap, Wild Oats, Hops, Gota Kola , or Kava kava.