Melatonin also increases your pain treshold at high doses (4 to 8 mg, maybe), most likely as an attempt by your body to make it easier to fall asleep at nighttime despite having something causing pain. This is actually more notable than its hypnotic effect, and you can use it to potentiate opioid or anti-inflammatory painkillers. The problem is the short half life (30 min) of melatonin, so it's only practical for short term pain such as a migraine headache.
As another effect, melatonin makes you tolerate more ionizing radiation without developing radiation sickness, and probably its direct metabolites cause this too. Other similar acting "natural products" can be found by Googling "radioprotective phytochemical", if you have a survivalist attitude and are interested in this.