Less is more with melatonin, over 1 mg and it can have a reverse effect, downregulating the M1 and M2 receptors it activates at lower levels (ie it stops them responding to more melatonin, they switch off). This is one reason why people find it ineffective or to cause undesirable side effects, the doses that are sold over the counter can often be far in excess of a therapeutic dose.

I occasionally supplement it with promethazine which is similar action to benadryl (which is harder to get in the UK) - it is more powerful as a sedative - but both should be used with caution as they have anticholinergic effects with long term use, I wouldn't use it every night.