Melatonin is not a herb per se and should be used with caution.
In fact it was not available as a supplement in the UK last year - did you get it from the UK? - Melatonin is one of the substances which helps to regulate sleep patterns - increasing the levels by supplementing can help promote sleep but can also lead to the brain expecting elevated levels of the chemical before triggering sleep (i.e. dependancy formation). I have used it before in one-off doses to help reset my body clock wen travelling long-haul and had no side effects.
Kava Kava was bulled from UK shelves not log ago after reports that it may cause kidney damage (I think - it was definitely puled though) - it has been cleared of thta is is now available again but some might say no smioke without fire...
In any case I would suggest trying each on their own for a period before combining them. Calcium supplements or dairy produce may help as well. I suffered terribly from insomnia for quite a while and managed to resolve it by adjusting my evening activity to accomodate a wind-down period at the end of the day and changing my diet. Let me know if you would like some more advice.